diff --git a/resources/celerity/map.php b/resources/celerity/map.php
--- a/resources/celerity/map.php
+++ b/resources/celerity/map.php
@@ -13,7 +13,7 @@
     'core.pkg.js' => '0f87a6eb',
     'darkconsole.pkg.js' => '1f9a31bc',
     'differential.pkg.css' => 'ea471cb0',
-    'differential.pkg.js' => '4a466790',
+    'differential.pkg.js' => '31e1b646',
     'diffusion.pkg.css' => 'b93d9b8c',
     'diffusion.pkg.js' => '84c8f8fd',
     'favicon.ico' => '30672e08',
@@ -390,9 +390,9 @@
     'rsrc/js/application/dashboard/behavior-dashboard-move-panels.js' => '408bf173',
     'rsrc/js/application/dashboard/behavior-dashboard-query-panel-select.js' => '453c5375',
     'rsrc/js/application/dashboard/behavior-dashboard-tab-panel.js' => 'd4eecc63',
-    'rsrc/js/application/diff/DiffChangeset.js' => '68758d99',
+    'rsrc/js/application/diff/DiffChangeset.js' => 'ec32b333',
     'rsrc/js/application/diff/DiffChangesetList.js' => '796922e0',
-    'rsrc/js/application/diff/DiffInline.js' => '1afe9760',
+    'rsrc/js/application/diff/DiffInline.js' => '3337c065',
     'rsrc/js/application/diff/behavior-preview-link.js' => '051c7832',
     'rsrc/js/application/differential/behavior-comment-preview.js' => 'b064af76',
     'rsrc/js/application/differential/behavior-diff-radios.js' => 'e1ff79b1',
@@ -777,9 +777,9 @@
     'phabricator-darklog' => 'c8e1ffe3',
     'phabricator-darkmessage' => 'c48cccdd',
     'phabricator-dashboard-css' => 'fe5b1869',
-    'phabricator-diff-changeset' => '68758d99',
+    'phabricator-diff-changeset' => 'ec32b333',
     'phabricator-diff-changeset-list' => '796922e0',
-    'phabricator-diff-inline' => '1afe9760',
+    'phabricator-diff-inline' => '3337c065',
     'phabricator-drag-and-drop-file-upload' => '58dea2fa',
     'phabricator-draggable-list' => 'bea6e7f4',
     'phabricator-fatal-config-template-css' => '8f18fa41',
@@ -1028,9 +1028,6 @@
       'javelin-util',
       'phabricator-keyboard-shortcut-manager',
     ),
-    '1afe9760' => array(
-      'javelin-dom',
-    ),
     '1bd28176' => array(
       'javelin-install',
       'javelin-dom',
@@ -1130,6 +1127,9 @@
       'javelin-dom',
       'javelin-workflow',
     ),
+    '3337c065' => array(
+      'javelin-dom',
+    ),
     '3ab51e2c' => array(
       'javelin-behavior',
       'javelin-behavior-device',
@@ -1403,17 +1403,6 @@
       'javelin-dom',
       'phabricator-notification',
     ),
-    '68758d99' => array(
-      'javelin-dom',
-      'javelin-util',
-      'javelin-stratcom',
-      'javelin-install',
-      'javelin-workflow',
-      'javelin-router',
-      'javelin-behavior-device',
-      'javelin-vector',
-      'phabricator-diff-inline',
-    ),
     '6882e80a' => array(
       'javelin-dom',
     ),
@@ -2142,6 +2131,17 @@
       'javelin-dom',
       'phabricator-draggable-list',
     ),
+    'ec32b333' => array(
+      'javelin-dom',
+      'javelin-util',
+      'javelin-stratcom',
+      'javelin-install',
+      'javelin-workflow',
+      'javelin-router',
+      'javelin-behavior-device',
+      'javelin-vector',
+      'phabricator-diff-inline',
+    ),
     'eded9ee8' => array(
       'javelin-behavior',
       'javelin-typeahead-ondemand-source',
diff --git a/webroot/rsrc/js/application/diff/DiffChangeset.js b/webroot/rsrc/js/application/diff/DiffChangeset.js
--- a/webroot/rsrc/js/application/diff/DiffChangeset.js
+++ b/webroot/rsrc/js/application/diff/DiffChangeset.js
@@ -581,8 +581,16 @@
     },
 
     getInlineByID: function(id) {
+      return this._queryInline('id', id);
+    },
+
+    getInlineByPHID: function(phid) {
+      return this._queryInline('phid', phid);
+    },
+
+    _queryInline: function(field, value) {
       // First, look for the inline in the objects we've already built.
-      var inline = this._findInlineByID(id);
+      var inline = this._findInline(field, value);
       if (inline) {
         return inline;
       }
@@ -590,13 +598,24 @@
       // If we haven't found a matching inline yet, rebuild all the inlines
       // present in the document, then look again.
       this._rebuildAllInlines();
-      return this._findInlineByID(id);
+      return this._findInline(field, value);
     },
 
-    _findInlineByID: function(id) {
+    _findInline: function(field, value) {
       for (var ii = 0; ii < this._inlines.length; ii++) {
         var inline = this._inlines[ii];
-        if (inline.getID() == id) {
+
+        var target;
+        switch (field) {
+          case 'id':
+            target = inline.getID();
+            break;
+          case 'phid':
+            target = inline.getPHID();
+            break;
+        }
+
+        if (target == value) {
           return inline;
         }
       }
diff --git a/webroot/rsrc/js/application/diff/DiffInline.js b/webroot/rsrc/js/application/diff/DiffInline.js
--- a/webroot/rsrc/js/application/diff/DiffInline.js
+++ b/webroot/rsrc/js/application/diff/DiffInline.js
@@ -63,6 +63,8 @@
         (this.getDisplaySide() == 'right') ||
         (data.left != data.right);
 
+      this._replyToCommentPHID = data.replyToCommentPHID;
+
       this.setInvisible(false);
 
       return this;
@@ -100,11 +102,45 @@
 
       this._replyToCommentPHID = inline._phid;
 
-      // TODO: This should insert correctly into the thread, not just at the
-      // bottom.
+      var changeset = this.getChangeset();
+
+      // We're going to figure out where in the document to position the new
+      // inline. Normally, it goes after any existing inline rows (so if
+      // several inlines reply to the same line, they appear in chronological
+      // order).
+
+      // However: if inlines are threaded, we want to put the new inline in
+      // the right place in the thread. This might be somewhere in the middle,
+      // so we need to do a bit more work to figure it out.
+
+      // To find the right place in the thread, we're going to look for any
+      // inline which is at or above the level of the comment we're replying
+      // to. This means we've reached a new fork of the thread, and should
+      // put our new inline before the comment we found.
+      var ancestor_map = {};
+      var ancestor = inline;
+      var reply_phid;
+      while (ancestor) {
+        reply_phid = ancestor.getReplyToCommentPHID();
+        if (!reply_phid) {
+          break;
+        }
+        ancestor_map[reply_phid] = true;
+        ancestor = changeset.getInlineByPHID(reply_phid);
+      }
+
       var parent_row = inline._row;
       var target_row = parent_row.nextSibling;
       while (target_row && JX.Stratcom.hasSigil(target_row, 'inline-row')) {
+        var target = changeset.getInlineForRow(target_row);
+        reply_phid = target.getReplyToCommentPHID();
+
+        // If we found an inline which is replying directly to some ancestor
+        // of this new comment, this is where the new rows go.
+        if (ancestor_map.hasOwnProperty(reply_phid)) {
+          break;
+        }
+
         target_row = target_row.nextSibling;
       }
 
@@ -318,6 +354,10 @@
       return this._id;
     },
 
+    getPHID: function() {
+      return this._phid;
+    },
+
     getChangesetID: function() {
       return this._changesetID;
     },