The structure of synaptobrevin, an intrinsic membrane protein of small synaptic vesicles from mammalian brain, was studied by purification and molecular cloning. Its message in bovine brain encodes a 116 amino acid protein whose sequence reveals it to be the mammalian homolog of Torpedo VAMP-1. Antibody probing demonstrates that the protein is also present in Drosophila, and its Drosophila homolog was cloned. Alignment of the sequences of synaptobrevin/VAMP-1 from the three species shows it to contain four domains, including a highly conserved central region of 63 amino acids that contains 75% invariant residues. The finding that a membrane protein from vertebrate synaptic vesicles is conserved in Drosophila points toward a central role of this protein in neurotransmission and should allow a genetic approach to neurotransmitter release.