diff --git a/apachesolr_multisitesearch.admin.inc b/apachesolr_multisitesearch.admin.inc
index 43b5156..a7116b9 100644
--- a/apachesolr_multisitesearch.admin.inc
+++ b/apachesolr_multisitesearch.admin.inc
@@ -13,7 +13,6 @@
  * this also reduces the clutter on the blocks admin page.
  */
 function apachesolr_multisitesearch_settings() {
-  module_load_include('inc', 'apachesolr_multisitesearch', 'apachesolr_multisitesearch.index');
   $form = array();
   $form['#tree'] = TRUE;
   $form['submit_message'] = array(
@@ -31,26 +30,7 @@ function apachesolr_multisitesearch_settings() {
     '#submit' => array('apachesolr_multisitesearch_refresh_metadata_now'),
   );
 
-  // Use the metadata and a list of all the hashes in the index
-  // to build up checkboxes for deleting site indexes.
-  // This is only necessary because sometimes hashes get
-  // stranded in the index and deleting the index from the normal
-  // admin screen doesn't rectify the problem.
-  $metadata = variable_get('apachesolr_multisitesearch_metadata', array());
-  $hashes = apachesolr_multisitesearch_get_site_hashes();
-  $options = array();
-  foreach ($hashes as $hash => $count) {
-    if ($hash == apachesolr_site_hash()) {
-      $options[$hash] = t('This site (!site, !count documents)', array('!site' => variable_get('site_name', 'Drupal'), '!count' => $count));
-    }
-    elseif (!empty($metadata[$hash])) {
-      $options[$hash] = $metadata[$hash]['site'] . ' ' . t('(!count documents)', array('!count' => $count));
-    }
-    else {
-      $options[$hash] = $hash . ' ' . t('(!count documents)', array('!count' => $count));
-    }
-  }
-
+  $options = apachesolr_multisitesearch_get_hash_options();
   if (count($options) > 0) {
     $form['admin']['delete']['hashes'] = array(
       '#type' => 'checkboxes',
diff --git a/apachesolr_multisitesearch.module b/apachesolr_multisitesearch.module
index e6514f4..6be1544 100644
--- a/apachesolr_multisitesearch.module
+++ b/apachesolr_multisitesearch.module
@@ -80,12 +80,29 @@ function apachesolr_multisitesearch_map_username($facets, $options) {
  */
 function apachesolr_multisitesearch_form_apachesolr_environment_edit_form_alter(&$form, &$form_state, $form_id) {
   $environment = reset($form_state['build_info']['args']);
-  $is_multisite = apachesolr_environment_variable_get($environment['env_id'], 'multisitesearch');
+
   $form['make_multisite'] = array(
     '#type' => 'checkbox',
     '#title' => t('Make this Solr search environment multisite capable'),
-    '#default_value' => $is_multisite,
+    '#default_value' => apachesolr_environment_variable_get($environment['env_id'], 'multisitesearch', FALSE),
   );
+
+  $options = apachesolr_multisitesearch_get_hash_options();
+  $form['multisitesearch_hashes'] = array(
+    '#type' => 'select',
+    '#title' => t('Limit searching to the following sites:'),
+    '#description' => t('If no sites selected, searching will be performed across all sites.'),
+    '#multiple' => TRUE,
+    '#options' => $options,
+    '#default_value' => apachesolr_environment_variable_get($environment['env_id'], 'multisitesearch_hashes', array()),
+    '#states' => array(
+      'visible' => array(
+        ':input[name="make_multisite"]' => array('checked' => TRUE),
+      ),
+    ),
+    '#access' => !empty($options),
+  );
+
   $form['actions']['save']['#submit'][] = 'apachesolr_multisitesearch_environment_edit_submit';
 }
 
@@ -95,6 +112,34 @@ function apachesolr_multisitesearch_form_apachesolr_environment_edit_form_alter(
 function apachesolr_multisitesearch_environment_edit_submit($form, &$form_state) {
   // Enable or disable multisite
   apachesolr_environment_variable_set($form_state['values']['env_id'], 'multisitesearch', $form_state['values']['make_multisite']);
+  apachesolr_environment_variable_set($form_state['values']['env_id'], 'multisitesearch_hashes', $form_state['values']['multisitesearch_hashes']);
+}
+
+/**
+ * Return an array of multisite search sites keyed by hash.
+ */
+function apachesolr_multisitesearch_get_hash_options() {
+  $options = &drupal_static(__FUNCTION__);
+
+  if (!isset($options)) {
+    module_load_include('inc', 'apachesolr_multisitesearch', 'apachesolr_multisitesearch.index');
+    $metadata = variable_get('apachesolr_multisitesearch_metadata', array());
+    $hashes = apachesolr_multisitesearch_get_site_hashes();
+    $options = array();
+    foreach ($hashes as $hash => $count) {
+      if ($hash == apachesolr_site_hash()) {
+        $options[$hash] = t('This site (!site, !count documents)', array('!site' => variable_get('site_name', 'Drupal'), '!count' => $count));
+      }
+      elseif (!empty($metadata[$hash])) {
+        $options[$hash] = $metadata[$hash]['site'] . ' ' . t('(!count documents)', array('!count' => $count));
+      }
+      else {
+        $options[$hash] = $hash . ' ' . t('(!count documents)', array('!count' => $count));
+      }
+    }
+  }
+
+  return $options;
 }
 
 /**
@@ -173,15 +218,17 @@ function apachesolr_multisitesearch_query_bundles() {
  */
 function apachesolr_multisitesearch_apachesolr_query_alter(DrupalSolrQueryInterface $query) {
   if (empty($query->multisite)) {
-    $env_id = $query->solr('getId');
-    $multisite = apachesolr_environment_variable_get($env_id, 'multisitesearch');
     // Add hash and site to our fields to retrieve
     $query->addParam('fl', 'hash');
     $query->addParam('fl', 'site');
-    if (empty($multisite)) {
+    $env_id = $query->solr('getId');
+    if (!apachesolr_environment_variable_get($env_id, 'multisitesearch', FALSE)) {
       // Limit single site searchs via the site hash.
       $query->addFilter('hash', apachesolr_site_hash());
     }
+    elseif ($hashes = apachesolr_environment_variable_get($env_id, 'multisitesearch_hashes', array())) {
+      $query->addFilter('hash', '(' . implode(' OR ', $hashes) . ')');
+    }
   }
 
   // Get the variable which contains the query exclusion keys.