diff --git a/src/applications/audit/application/PhabricatorAuditApplication.php b/src/applications/audit/application/PhabricatorAuditApplication.php
--- a/src/applications/audit/application/PhabricatorAuditApplication.php
+++ b/src/applications/audit/application/PhabricatorAuditApplication.php
@@ -19,9 +19,7 @@
   }
 
   public function canUninstall() {
-    // Audit was once a separate application, but has largely merged with
-    // Diffusion.
-    return false;
+    return true;
   }
 
   public function isPinnedByDefault(PhabricatorUser $viewer) {
diff --git a/src/applications/audit/conduit/AuditConduitAPIMethod.php b/src/applications/audit/conduit/AuditConduitAPIMethod.php
--- a/src/applications/audit/conduit/AuditConduitAPIMethod.php
+++ b/src/applications/audit/conduit/AuditConduitAPIMethod.php
@@ -4,7 +4,7 @@
 
   final public function getApplication() {
     return PhabricatorApplication::getByClass(
-      'PhabricatorDiffusionApplication');
+      'PhabricatorAuditApplication');
   }
 
 }
diff --git a/src/applications/audit/query/PhabricatorCommitSearchEngine.php b/src/applications/audit/query/PhabricatorCommitSearchEngine.php
--- a/src/applications/audit/query/PhabricatorCommitSearchEngine.php
+++ b/src/applications/audit/query/PhabricatorCommitSearchEngine.php
@@ -70,6 +70,7 @@
   }
 
   protected function buildCustomSearchFields() {
+    $show_audit_fields = (id(new PhabricatorAuditApplication())->isInstalled());
     $show_packages = PhabricatorApplication::isClassInstalled(
       'PhabricatorPackagesApplication');
     return array(
@@ -95,6 +96,7 @@
         ->setConduitKey('auditors')
         ->setAliases(array('auditor', 'auditors', 'auditorPHID'))
         ->setDatasource(new DiffusionAuditorFunctionDatasource())
+        ->setIsHidden(!$show_audit_fields)
         ->setDescription(
           pht(
             'Find commits where given users, projects, or packages are '.
@@ -106,6 +108,7 @@
         ->setOptions(DiffusionCommitAuditStatus::newOptions())
         ->setDeprecatedOptions(
           DiffusionCommitAuditStatus::newDeprecatedOptions())
+        ->setIsHidden(!$show_audit_fields)
         ->setDescription(pht('Find commits with given audit statuses.')),
       id(new PhabricatorSearchDatasourceField())
         ->setLabel(pht('Repositories'))
@@ -172,9 +175,13 @@
     $names = array();
 
     if ($this->requireViewer()->isLoggedIn()) {
-      $names['active'] = pht('Active Audits');
+      if (id(new PhabricatorAuditApplication())->isInstalled()) {
+        $names['active'] = pht('Active Audits');
+      }
       $names['authored'] = pht('Authored');
-      $names['audited'] = pht('Audited');
+      if (id(new PhabricatorAuditApplication())->isInstalled()) {
+        $names['audited'] = pht('Audited');
+      }
     }
 
     $names['all'] = pht('All Commits');
@@ -224,9 +231,11 @@
 
     $bucket = $this->getResultBucket($query);
 
+    // hide "Auditors" on /diffusion/commit/query/all/ if Audit not installed
+    $show_auditors = id(new PhabricatorAuditApplication())->isInstalled();
     $template = id(new DiffusionCommitGraphView())
       ->setViewer($viewer)
-      ->setShowAuditors(true);
+      ->setShowAuditors($show_auditors);
 
     $views = array();
     if ($bucket) {
diff --git a/src/applications/diffusion/editor/DiffusionCommitEditEngine.php b/src/applications/diffusion/editor/DiffusionCommitEditEngine.php
--- a/src/applications/diffusion/editor/DiffusionCommitEditEngine.php
+++ b/src/applications/diffusion/editor/DiffusionCommitEditEngine.php
@@ -100,19 +100,22 @@
     $data = $object->getCommitData();
 
     $fields = array();
-
-    $fields[] = id(new PhabricatorDatasourceEditField())
-      ->setKey('auditors')
-      ->setLabel(pht('Auditors'))
-      ->setDatasource(new DiffusionAuditorDatasource())
-      ->setUseEdgeTransactions(true)
-      ->setTransactionType(
-        DiffusionCommitAuditorsTransaction::TRANSACTIONTYPE)
-      ->setCommentActionLabel(pht('Change Auditors'))
-      ->setDescription(pht('Auditors for this commit.'))
-      ->setConduitDescription(pht('Change the auditors for this commit.'))
-      ->setConduitTypeDescription(pht('New auditors.'))
-      ->setValue($object->getAuditorPHIDsForEdit());
+    // remove "Change Auditors" from "Add Action" dropdown etc
+    // if Audit is not installed
+    if (id(new PhabricatorAuditApplication())->isInstalled()) {
+      $fields[] = id(new PhabricatorDatasourceEditField())
+        ->setKey('auditors')
+        ->setLabel(pht('Auditors'))
+          ->setDatasource(new DiffusionAuditorDatasource())
+        ->setUseEdgeTransactions(true)
+        ->setTransactionType(
+          DiffusionCommitAuditorsTransaction::TRANSACTIONTYPE)
+        ->setCommentActionLabel(pht('Change Auditors'))
+        ->setDescription(pht('Auditors for this commit.'))
+        ->setConduitDescription(pht('Change the auditors for this commit.'))
+        ->setConduitTypeDescription(pht('New auditors.'))
+        ->setValue($object->getAuditorPHIDsForEdit());
+    }
 
     $actions = DiffusionCommitActionTransaction::loadAllActions();
     $actions = msortv($actions, 'getCommitActionOrderVector');
diff --git a/src/applications/diffusion/herald/DiffusionAuditorsAddAuditorsHeraldAction.php b/src/applications/diffusion/herald/DiffusionAuditorsAddAuditorsHeraldAction.php
--- a/src/applications/diffusion/herald/DiffusionAuditorsAddAuditorsHeraldAction.php
+++ b/src/applications/diffusion/herald/DiffusionAuditorsAddAuditorsHeraldAction.php
@@ -9,8 +9,13 @@
     return pht('Add auditors');
   }
 
+  // hide "Add auditors" Herald action if Audit not installed
   public function supportsRuleType($rule_type) {
-    return ($rule_type != HeraldRuleTypeConfig::RULE_TYPE_PERSONAL);
+    if (id(new PhabricatorAuditApplication())->isInstalled()) {
+      return ($rule_type != HeraldRuleTypeConfig::RULE_TYPE_PERSONAL);
+    } else {
+      return false;
+    }
   }
 
   public function applyEffect($object, HeraldEffect $effect) {
diff --git a/src/applications/diffusion/herald/DiffusionAuditorsAddSelfHeraldAction.php b/src/applications/diffusion/herald/DiffusionAuditorsAddSelfHeraldAction.php
--- a/src/applications/diffusion/herald/DiffusionAuditorsAddSelfHeraldAction.php
+++ b/src/applications/diffusion/herald/DiffusionAuditorsAddSelfHeraldAction.php
@@ -9,8 +9,13 @@
     return pht('Add me as an auditor');
   }
 
+  // hide "Add me as an auditor" Herald action if Audit not installed
   public function supportsRuleType($rule_type) {
-    return ($rule_type == HeraldRuleTypeConfig::RULE_TYPE_PERSONAL);
+    if (id(new PhabricatorAuditApplication())->isInstalled()) {
+      return ($rule_type == HeraldRuleTypeConfig::RULE_TYPE_PERSONAL);
+    } else {
+      return false;
+    }
   }
 
   public function applyEffect($object, HeraldEffect $effect) {
diff --git a/src/applications/diffusion/herald/DiffusionCommitAuditorsHeraldField.php b/src/applications/diffusion/herald/DiffusionCommitAuditorsHeraldField.php
--- a/src/applications/diffusion/herald/DiffusionCommitAuditorsHeraldField.php
+++ b/src/applications/diffusion/herald/DiffusionCommitAuditorsHeraldField.php
@@ -5,6 +5,15 @@
 
   const FIELDCONST = 'diffusion.commit.auditors';
 
+  // hide "Auditors" Herald condition if Audit not installed
+  public function supportsObject($object) {
+    if (id(new PhabricatorAuditApplication())->isInstalled()) {
+      return ($object instanceof PhabricatorRepositoryCommit);
+    } else {
+      return false;
+    }
+  }
+
   public function getHeraldFieldName() {
     return pht('Auditors');
   }
diff --git a/src/applications/diffusion/herald/DiffusionCommitPackageAuditHeraldField.php b/src/applications/diffusion/herald/DiffusionCommitPackageAuditHeraldField.php
--- a/src/applications/diffusion/herald/DiffusionCommitPackageAuditHeraldField.php
+++ b/src/applications/diffusion/herald/DiffusionCommitPackageAuditHeraldField.php
@@ -5,6 +5,16 @@
 
   const FIELDCONST = 'diffusion.commit.package.audit';
 
+  // hide "Affected packages that need audit" Herald condition
+  // if Audit not installed
+  public function supportsObject($object) {
+    if (id(new PhabricatorAuditApplication())->isInstalled()) {
+      return ($object instanceof PhabricatorRepositoryCommit);
+    } else {
+      return false;
+    }
+  }
+
   public function getHeraldFieldName() {
     return pht('Affected packages that need audit');
   }
diff --git a/src/applications/diffusion/view/DiffusionCommitGraphView.php b/src/applications/diffusion/view/DiffusionCommitGraphView.php
--- a/src/applications/diffusion/view/DiffusionCommitGraphView.php
+++ b/src/applications/diffusion/view/DiffusionCommitGraphView.php
@@ -169,7 +169,10 @@
         $this->addBuildAction($item_view, $hash);
       }
 
-      $this->addAuditAction($item_view, $hash);
+      // hide Audit entry on /diffusion/commit/query/all if Audit not installed
+      if (id(new PhabricatorAuditApplication())->isInstalled()) {
+        $this->addAuditAction($item_view, $hash);
+      }
 
       if ($show_auditors) {
         $auditor_list = $item_view->newMapView();
diff --git a/src/applications/diffusion/xaction/DiffusionCommitActionTransaction.php b/src/applications/diffusion/xaction/DiffusionCommitActionTransaction.php
--- a/src/applications/diffusion/xaction/DiffusionCommitActionTransaction.php
+++ b/src/applications/diffusion/xaction/DiffusionCommitActionTransaction.php
@@ -8,6 +8,9 @@
   }
 
   public function isActionAvailable($object, PhabricatorUser $viewer) {
+    if (!id(new PhabricatorAuditApplication())->isInstalled()) {
+      return false;
+    }
     try {
       $this->validateAction($object, $viewer);
       return true;