diff --git a/src/unit/engine/PhutilUnitTestEngine.php b/src/unit/engine/PhutilUnitTestEngine.php
--- a/src/unit/engine/PhutilUnitTestEngine.php
+++ b/src/unit/engine/PhutilUnitTestEngine.php
@@ -17,16 +17,21 @@
     }
 
     if (!$run_tests) {
-      throw new ArcanistNoEffectException('No tests to run.');
+      throw new ArcanistNoEffectException(pht('No tests to run.'));
     }
 
     $enable_coverage = $this->getEnableCoverage();
+
     if ($enable_coverage !== false) {
       if (!function_exists('xdebug_start_code_coverage')) {
         if ($enable_coverage === true) {
           throw new ArcanistUsageException(
-            'You specified --coverage but xdebug is not available, so '.
-            'coverage can not be enabled for PhutilUnitTestEngine.');
+            pht(
+              'You specified %s but %s is not available, so '.
+              'coverage can not be enabled for %s.',
+              '--coverage',
+              'XDebug',
+              __CLASS__));
         }
       } else {
         $enable_coverage = true;
@@ -125,31 +130,27 @@
       }
 
       $path = Filesystem::resolvePath($path, $project_root);
+      $library_path = Filesystem::readablePath($path, $library_root);
 
-      if (!is_dir($path)) {
-        $path = dirname($path);
-      }
-
-      if ($path == $library_root) {
-        $look_here[$library_name.':.'] = array(
-          'library' => $library_name,
-          'path'    => '',
-        );
-      } else if (!Filesystem::isDescendant($path, $library_root)) {
+      if (!Filesystem::isDescendant($path, $library_root)) {
         // We have encountered some kind of symlink maze -- for instance, $path
         // is some symlink living outside the library that links into some file
         // inside the library. Just ignore these cases, since the affected file
         // does not actually lie within the library.
         continue;
+      } else if (is_file($path) && preg_match('/\b__tests__\b/', $path)) {
+        $look_here[$library_name.':'.$library_path] = array(
+          'library' => $library_name,
+          'path'    => $library_path,
+        );
       } else {
-        $library_path = Filesystem::readablePath($path, $library_root);
         do {
           $look_here[$library_name.':'.$library_path] = array(
             'library' => $library_name,
-            'path'    => $library_path,
+            'path'    => $library_path == $library_root ? '' : $library_path,
           );
           $library_path = dirname($library_path);
-        } while ($library_path != '.');
+        } while ($library_path != '.' && $library_path == $library_root);
       }
     }
 
@@ -170,7 +171,7 @@
       $symbols = id(new PhutilSymbolLoader())
         ->setType('class')
         ->setLibrary($library)
-        ->setPathPrefix(($path ? $path.'/' : '').'__tests__/')
+        ->setPathPrefix($path)
         ->setAncestorClass('ArcanistPhutilTestCase')
         ->setConcreteOnly(true)
         ->selectAndLoadSymbols();