diff --git a/src/MutableMap/MutableMap.cs b/src/MutableMap/MutableMap.cs
new file mode 100644
index 00000000..ff5358b1
--- /dev/null
+++ b/src/MutableMap/MutableMap.cs
@@ -0,0 +1,78 @@
+/*******************************************************************************
+*  Copyright by the contributors to the Dafny Project
+*  SPDX-License-Identifier: MIT
+*******************************************************************************/
+using System.Collections;
+using System.Collections.Concurrent;
+using System.Collections.Immutable;
+using System.Numerics;
+using _System;
+using Dafny;
+
+
+namespace DafnyLibraries {
+   public partial class MutableMap<K, V>
+   {
+      private ConcurrentDictionary<K, V> m;
+      public MutableMap() {
+         m = new ConcurrentDictionary<K, V>();
+      }
+
+      public IMap<K, V> content() {
+         var keyPairs = new List<IPair<K, V>>();
+         foreach (var k in m.Keys) {
+            keyPairs.Add(new Pair<K, V>(k, m[k]));
+         }
+         return Map<K, V>.FromCollection(keyPairs);
+      }
+
+      public void Put(K k, V v)
+      {
+         m[k] = v;
+      }
+
+      public Dafny.ISet<K> Keys()
+      {
+         var keys = m.Keys;
+         return Set<K>.FromCollection(keys);
+      }
+
+      public bool HasKey(K k)
+      {
+         return m.ContainsKey(k);
+      }
+
+      public Dafny.ISet<V> Values()
+      {
+         var values= m.Values;
+         return Set<V>.FromCollection(values);
+      }
+
+      public Dafny.ISet<_ITuple2<K, V>> Items()
+      {
+         var keyPairs = new List<_ITuple2<K, V>>();
+         foreach (var k in m.Keys) {
+            keyPairs.Add(new Tuple2<K, V>(k, m[k]));
+         }
+         return Set<_ITuple2<K,V>>.FromCollection(keyPairs);
+      }
+
+      public V Select(K k)
+      {
+         return m[k];
+      }
+
+      public void Remove(K k)
+      {
+         if (HasKey(k)) {
+            var keypair = new KeyValuePair<K, V>(k, m[k]); 
+            m.TryRemove(keypair);
+         }
+      }
+
+      public BigInteger Size()
+      {
+         return new BigInteger(m.Count);
+      }
+   }
+}