diff --git a/lib/ace/document.js b/lib/ace/document.js
index c689553c..9d6aaac3 100644
--- a/lib/ace/document.js
+++ b/lib/ace/document.js
@@ -261,9 +261,13 @@ var Document = function(textOrLines) {
             throw errorText;
         }
         
+        // Validate action.
+        if (delta.action != 'insert' && delta.action != 'delete')
+            fnThrow('Delta action must be "insert" or "delete".');
+        
         // Validate lines.
         if (!delta.lines instanceof Array)
-            fnThrow('Delta object lines must be an array');
+            fnThrow('Delta lines must be an array');
         
         // Validate range type.
         if (!delta.range instanceof Range)
@@ -567,30 +571,50 @@ var Document = function(textOrLines) {
         this.$validateDelta(delta);
         
         // Apply delta.
-        switch (delta.action)
+        if (delta.range.start.row == delta.range.end.row)
         {
-            case 'insert':
-                splitLine(this.$lines, delta.range.start);
-                for (var i = 0; i < delta.lines.length; i++) {
-                    var row = delta.range.start.row + 1 + i;
-                    this.$lines.splice(row, 0, delta.lines[i]);
-                }
-                joinLineWithNext(this.$lines, delta.range.start.row);
-                joinLineWithNext(this.$lines, delta.range.end.row);
-                break;
+            // Apply single-line delta.
+            // Note: The multi-line code below correctly handle single-line
+            //       deltas too, but we need to short-circuit for speed.
+            var row         = delta.range.start.row;
+            var startColumn = delta.range.start.column;
+            var endColumn   = delta.range.end.column;
+            var line        = this.$lines[row];
+            switch (delta.action) {
+                
+                case 'insert':
+                    this.$lines[row] = line.substring(0, startColumn) + delta.lines[0] + line.substring(startColumn);
+                    break;
+                
+                case 'delete':
+                    this.$lines[row] = line.substring(0, startColumn) + line.substring(endColumn);
+                    break;
+            }
+        } else {
             
-            case 'delete':                
-                splitLine(this.$lines, delta.range.end);
-                splitLine(this.$lines, delta.range.start);
-                this.$lines.splice(
-                    delta.range.start.row + 1,                       // Where to start deleting
-                    delta.range.end.row - delta.range.start.row + 1  // Num lines to delete.
-                );
-                joinLineWithNext(this.$lines, delta.range.start.row);
-                break;
-            
-            default:
-                throw 'Invalid delta type: ' + delta.action
+            // Apply multi-line delta.
+            switch (delta.action) {
+                
+                case 'insert':
+                    splitLine(this.$lines, delta.range.start);
+                    for (var i = 0; i < delta.lines.length; i++) {
+                        var row = delta.range.start.row + 1 + i;
+                        this.$lines.splice(row, 0, delta.lines[i]);
+                    }
+                    joinLineWithNext(this.$lines, delta.range.start.row);
+                    joinLineWithNext(this.$lines, delta.range.end.row);
+                    break;
+                
+                case 'delete':                
+                    splitLine(this.$lines, delta.range.end);
+                    splitLine(this.$lines, delta.range.start);
+                    this.$lines.splice(
+                        delta.range.start.row + 1,                       // Where to start deleting
+                        delta.range.end.row - delta.range.start.row + 1  // Num lines to delete.
+                    );
+                    joinLineWithNext(this.$lines, delta.range.start.row);
+                    break;
+            }
         }
         
         this._emit("change", { data: delta });