The synapse-associated proteins SAP97 and SAP102 are mammalian proteins that are structurally related to the Drosophila tumor suppressor protein DlgA. Previous analyses revealed that DlgA is essential for the integrity of epithelia and neuromuscular synapses. Here we show that synaptic bouton structure is severely affected in mutant larvae carrying the dlg-1(XI-2) allele. We have tested SAP97 and SAP102 for functional homology to DlgA by heterologous expression in Drosophila. Both SAP97 and SAP102 can suppress tumor formation in dlg-1 mutant flies and mimic DlgA at larval neuromuscular junctions. Neuronal expression of SAP97 or SAP102 is required for morphological restoration of synaptic boutons, indicating that presynaptic DlgA function is essential for establishing structurally intact motor nerve terminals at larval neuromuscular junctions.