Index: src/test/org/apache/lucene/search/TestMultiSearcher.java
===================================================================
--- src/test/org/apache/lucene/search/TestMultiSearcher.java	(revision 531709)
+++ src/test/org/apache/lucene/search/TestMultiSearcher.java	(working copy)
@@ -354,4 +354,42 @@
         ramDirectory1.close();
         ramDirectory2.close();
     }
+    
+    /**
+     * test that custom similarity is in effect when using MultiSearcher (LUCENE-789).
+     * @throws IOException 
+     */
+    public void testCustomSimilarity () throws IOException {
+        RAMDirectory dir = new RAMDirectory();
+        initIndex(dir, 10, true, "x"); // documents with two tokens "doc0" and "x", "doc1" and x, etc...
+        IndexSearcher srchr = new IndexSearcher(dir);
+        MultiSearcher msrchr = getMultiSearcherInstance(new Searcher[]{srchr});
+        
+        Similarity customSimilarity = new DefaultSimilarity() {
+            // overide all
+            public float idf(int docFreq, int numDocs) { return 100.0f; }
+            public float coord(int overlap, int maxOverlap) { return 1.0f; }
+            public float lengthNorm(String fieldName, int numTokens) { return 1.0f; }
+            public float queryNorm(float sumOfSquaredWeights) { return 1.0f; }
+            public float sloppyFreq(int distance) { return 1.0f; }
+            public float tf(float freq) { return 1.0f; }
+        };
+        
+        srchr.setSimilarity(customSimilarity);
+        msrchr.setSimilarity(customSimilarity);
+  
+        Query query=new TermQuery(new Term("contents", "doc0"));
+  
+        // Get a score from IndexSearcher
+        TopDocs topDocs = srchr.search(query, null, 1);
+        float score1 = topDocs.getMaxScore();
+        
+        // Get the score from MultiSearcher
+        topDocs = msrchr.search(query, null, 1);
+        float scoreN = topDocs.getMaxScore();
+        
+        // The scores from the IndexSearcher and Multisearcher should be the same
+        // if the same similarity is used.
+        assertEquals("MultiSearcher score must be equal to single esrcher score!", score1, scoreN, 1e-6);
+    }
 }
Index: src/java/org/apache/lucene/search/MultiSearcher.java
===================================================================
--- src/java/org/apache/lucene/search/MultiSearcher.java	(revision 531709)
+++ src/java/org/apache/lucene/search/MultiSearcher.java	(working copy)
@@ -43,9 +43,10 @@
     private Map dfMap; // Map from Terms to corresponding doc freqs
     private int maxDoc; // document count
 
-    public CachedDfSource(Map dfMap, int maxDoc) {
+    public CachedDfSource(Map dfMap, int maxDoc, Similarity similarity) {
       this.dfMap = dfMap;
       this.maxDoc = maxDoc;
+      setSimilarity(similarity);
     }
 
     public int docFreq(Term term) {
@@ -106,7 +107,7 @@
     public TopFieldDocs search(Weight weight,Filter filter,int n,Sort sort) {
       throw new UnsupportedOperationException();
     }
-  };
+  }
 
 
   private Searchable[] searchables;
@@ -320,7 +321,7 @@
 
     // step4
     int numDocs = maxDoc();
-    CachedDfSource cacheSim = new CachedDfSource(dfMap, numDocs);
+    CachedDfSource cacheSim = new CachedDfSource(dfMap, numDocs, getSimilarity());
 
     return rewrittenQuery.weight(cacheSim);
   }