In this paper an interconnection architecture for a large scale multicast ATM switch [LSMA] is introduced. The proposed architecture permits the modular growth of its size from a small number of ports to very large switch sizes. The expansion of the switch size is implemented in a smoothing way with simple modification of already existing connections. An important feature of LSMA is that the system growth is independent of the number of vertical stages, i.e. the system always contains three stages only. The general description of the input switch module which contains the multicast network is also introduced. Comparison between the number of middle- stage modules and the number of overall switch modules for different architectures is presented.