diff --git a/db/repl/rs.h b/db/repl/rs.h
index 6f61d113d0a..8677f91638a 100644
--- a/db/repl/rs.h
+++ b/db/repl/rs.h
@@ -111,10 +111,13 @@ namespace mongo {
         /**
          * This is a cache of ghost slaves
          */
-        map<mongo::OID,GhostSlave> _ghostCache;
+        typedef map<mongo::OID,GhostSlave> MAP;
+        MAP _ghostCache;
+        RWLock _lock; // protects _ghostCache
         ReplSetImpl *rs;
+
     public:
-        GhostSync(ReplSetImpl *_rs) : task::Server("rs ghost sync"), rs(_rs) {}
+        GhostSync(ReplSetImpl *_rs) : task::Server("rs ghost sync"), _lock("GhostSync"), rs(_rs) {}
         ~GhostSync() {
             log() << "~GhostSync() called" << rsLog;
         }
diff --git a/db/repl/rs_sync.cpp b/db/repl/rs_sync.cpp
index 5850b22ce59..ec0131afad5 100644
--- a/db/repl/rs_sync.cpp
+++ b/db/repl/rs_sync.cpp
@@ -512,8 +512,10 @@ namespace mongo {
         cc().shutdown();
     }
 
-    void GhostSync::associateSlave(const BSONObj& rid, const int memberId) {
-        GhostSlave &slave = _ghostCache[rid["_id"].OID()];
+    void GhostSync::associateSlave(const BSONObj& id, const int memberId) {
+        const OID rid = id["_id"].OID();
+        rwlock lk( _lock , true );
+        GhostSlave &slave = _ghostCache[rid];
         if (slave.init) {
             LOG(1) << "tracking " << slave.slave->h().toString() << " as " << rid << rsLog;
             return;
@@ -529,66 +531,82 @@ namespace mongo {
         }
     }
 
-    void GhostSync::updateSlave(const mongo::OID& id, const OpTime& last) {
-        GhostSlave& slave = _ghostCache[id];
+    void GhostSync::updateSlave(const mongo::OID& rid, const OpTime& last) {
+        rwlock lk( _lock , false );
+        MAP::iterator i = _ghostCache.find( rid );
+        if ( i == _ghostCache.end() ) {
+            OCCASIONALLY log() << "couldn't update slave " << rid << " no entry" << rsLog;
+            return;
+        }
+        
+        GhostSlave& slave = i->second;
         if (!slave.init) {
-            OCCASIONALLY log() << "couldn't update slave " << id << rsLog;
+            OCCASIONALLY log() << "couldn't update slave " << rid << " not init" << rsLog;            
             return;
         }
 
         ((ReplSetConfig::MemberCfg)slave.slave->config()).updateGroups(last);
     }
 
-    void GhostSync::percolate(const BSONObj& rid, const OpTime& last) {
-        GhostSlave &s = _ghostCache[rid["_id"].OID()];
-        if (!s.init) {
-            log() << "replSet couldn't find a slave with id " << rid
-                  << ", not faux syncing" << rsLog;
+    void GhostSync::percolate(const BSONObj& id, const OpTime& last) {
+        const OID rid = id["_id"].OID();
+        rwlock lk( _lock , false );
+
+        MAP::iterator i = _ghostCache.find( rid );
+        if ( i == _ghostCache.end() ) {
+            OCCASIONALLY log() << "couldn't percolate slave " << rid << " no entry" << rsLog;
             return;
         }
-        assert(s.slave);
+        
+        GhostSlave& slave = i->second;
+        if (!slave.init) {
+            OCCASIONALLY log() << "couldn't percolate slave " << rid << " not init" << rsLog;            
+            return;
+        }
+
+        assert(slave.slave);
 
         const Member *target = rs->_currentSyncTarget;
         if (!target || rs->box.getState().primary()
             // we are currently syncing from someone who's syncing from us
             // the target might end up with a new Member, but s.slave never
             // changes so we'll compare the names
-            || target == s.slave || target->fullName() == s.slave->fullName()) {
+            || target == slave.slave || target->fullName() == slave.slave->fullName()) {
             log(1) << "replica set ghost target no good" << endl;
             return;
         }
 
         try {
-            if (!s.reader.haveCursor()) {
-                if (!s.reader.connect(rid, s.slave->id(), target->fullName())) {
+            if (!slave.reader.haveCursor()) {
+                if (!slave.reader.connect(id, slave.slave->id(), target->fullName())) {
                     // error message logged in OplogReader::connect
                     return;
                 }
-                s.reader.ghostQueryGTE(rsoplog, last);
+                slave.reader.ghostQueryGTE(rsoplog, last);
             }
 
-            log(1) << "last: " << s.last.toString() << " to " << last.toString() << rsLog;
+            log(1) << "last: " << slave.last.toString() << " to " << last.toString() << rsLog;
             
-            if (s.last > last) {
+            if (slave.last > last) {
                 return;
             }
             
-            while (s.last <= last) {
-                if (!s.reader.more()) {
+            while (slave.last <= last) {
+                if (!slave.reader.more()) {
                     // we'll be back
                     return;
                 }
             
-                BSONObj o = s.reader.nextSafe();
-                s.last = o["ts"]._opTime();
+                BSONObj o = slave.reader.nextSafe();
+                slave.last = o["ts"]._opTime();
             }
-            log(2) << "now last is " << s.last.toString() << rsLog;
+            log(2) << "now last is " << slave.last.toString() << rsLog;
         }
         catch (DBException& e) {
             // we'll be back
             log(2) << "replSet ghost sync error: " << e.what() << " for "
-                   << s.slave->fullName() << rsLog;
-            s.reader.resetConnection();
+                   << slave.slave->fullName() << rsLog;
+            slave.reader.resetConnection();
         }
     }
 }