diff --git a/Changes.md b/Changes.md
index 6e6ad0b6a4..79d9b20e94 100644
--- a/Changes.md
+++ b/Changes.md
@@ -1,6 +1,11 @@
 1.4.x.x (relative to 1.4.14.0)
 =======
 
+Improvements
+------------
+
+- Arnold : Added support for Int64Data and UInt64Data custom attributes, allowing USD's `instanceId` to be used as a custom attribute in the Instancer node. Warnings are emitted if values are out of range for Arnold's 32 bit ints.
+
 Fixes
 -----
 
diff --git a/python/IECoreArnoldTest/ParameterAlgoTest.py b/python/IECoreArnoldTest/ParameterAlgoTest.py
index 0f189affc9..0fe1cf6fb1 100644
--- a/python/IECoreArnoldTest/ParameterAlgoTest.py
+++ b/python/IECoreArnoldTest/ParameterAlgoTest.py
@@ -144,5 +144,23 @@ def testVectorIntData( self ) :
 			IECoreArnold.ParameterAlgo.setParameter( n, "customV3i", IECore.V3iData( imath.V3i( 3, 4, 5 ) ) )
 			self.assertEqual( arnold.AiNodeGetVec( n, "customV3i" ), arnold.AtVector( 3, 4, 5 ) )
 
+	def testInt64Data( self ) :
+
+		with IECoreArnold.UniverseBlock( writable = True ) as universe :
+
+			n = arnold.AiNode( universe, "ginstance" )
+			with IECore.CapturingMessageHandler() as mh :
+				IECoreArnold.ParameterAlgo.setParameter( n, "customInt64", IECore.Int64Data( 1 ) )
+				IECoreArnold.ParameterAlgo.setParameter( n, "customUInt64", IECore.UInt64Data( 2 ) )
+				IECoreArnold.ParameterAlgo.setParameter( n, "customInt64OutOfRange", IECore.Int64Data( 2 ** 31 ) )
+				IECoreArnold.ParameterAlgo.setParameter( n, "customUInt64OutOfRange", IECore.UInt64Data( 2 ** 32 ) )
+
+			self.assertEqual( arnold.AiNodeGetInt( n, "customInt64" ), 1 )
+			self.assertEqual( arnold.AiNodeGetUInt( n, "customUInt64" ), 2 )
+
+			self.assertEqual( len( mh.messages ), 2 )
+			self.assertEqual( mh.messages[0].message, 'Int64Data value 2147483648 is out of range for parameter "customInt64OutOfRange"' )
+			self.assertEqual( mh.messages[1].message, 'UInt64Data value 4294967296 is out of range for parameter "customUInt64OutOfRange"' )
+
 if __name__ == "__main__":
 	unittest.main()
diff --git a/src/IECoreArnold/ParameterAlgo.cpp b/src/IECoreArnold/ParameterAlgo.cpp
index b00de67793..346a7b2998 100644
--- a/src/IECoreArnold/ParameterAlgo.cpp
+++ b/src/IECoreArnold/ParameterAlgo.cpp
@@ -90,13 +90,37 @@ void setParameterInternal( AtNode *node, AtString name, int parameterType, bool
 		switch( parameterType )
 		{
 			case AI_TYPE_INT :
-				if( const IntData *data = dataCast<IntData>( name, value ) )
+				if( const Int64Data *data = runTimeCast<const Int64Data>( value ) )
+				{
+					int i = static_cast<int>( data->readable() );
+					if( i == data->readable() )
+					{
+						AiNodeSetInt( node, name, i );
+					}
+					else
+					{
+						msg( Msg::Warning, "setParameter", fmt::format( "Int64Data value {} is out of range for parameter \"{}\"", data->readable(), name ) );
+					}
+				}
+				else if( const IntData *data = dataCast<IntData>( name, value ) )
 				{
 					AiNodeSetInt( node, name, data->readable() );
 				}
 				break;
 			case AI_TYPE_UINT :
-				if( const IntData *data = runTimeCast<const IntData>( value ) )
+				if( const UInt64Data *data = runTimeCast<const UInt64Data>( value ) )
+				{
+					unsigned int i = static_cast<unsigned int>( data->readable() );
+					if( i == data->readable() )
+					{
+						AiNodeSetUInt( node, name, i );
+					}
+					else
+					{
+						msg( Msg::Warning, "setParameter", fmt::format( "UInt64Data value {} is out of range for parameter \"{}\"", data->readable(), name ) );
+					}
+				}
+				else if( const IntData *data = runTimeCast<const IntData>( value ) )
 				{
 					AiNodeSetUInt( node, name, std::max( 0, data->readable() ) );
 				}
@@ -459,9 +483,11 @@ int parameterType( IECore::TypeId dataType, bool &array )
 		// non-array types
 
 		case IntDataTypeId :
+		case Int64DataTypeId :
 			array = false;
 			return AI_TYPE_INT;
 		case UIntDataTypeId :
+		case UInt64DataTypeId :
 			array = false;
 			return AI_TYPE_UINT;
 		case FloatDataTypeId :