The voltage-activated Na channel (NaCh) is an integral membrane protein that is enriched at the neuromuscular end plate. Using loose-patch voltage-clamp and immunofluorescence, we have found that the aggregation of NaChs occurs late, during maturation of the neuromuscular junction. A decline in expression of embryonic NaCh mRNA and increase in adult NaCh mRNA precedes the onset of aggregation, and the appearance of functional adult NaChs coincides with NaCh aggregation. We tested the possibility that only the adult NaCh subtype could aggregate during development and found that both the embryonic and adult isoforms become concentrated at the synapse. The NaCh is the first postsynaptic membrane protein shown to become clustered postnatally, and the mechanism producing this aggregation appears to be different from the process producing aggregation of other synaptic proteins.