Synapsin I is a peripheral membrane protein of synaptic vesicles that mediates their attachment to the cytoskeleton. Human genomic clones containing the full coding sequence of synapsin I were isolated, and the exons were mapped and sequenced. Human synapsin I is encoded by a single copy gene containing 13 exons ranging in size from 58 base pairs to more than 1 kilobase that are unequally distributed over more than 30 kilobases of DNA on the X-chromosome. The differential splicing of the primary synapsin I transcript that generates synapsins Ia and Ib involves alternative use of splice acceptor sites at the last intron-exon boundary. The primary structure of synapsin I is highly conserved between the human, rat, and bovine proteins (95% identity). The intron placement within that primary structure correlates with the previously postulated domain model of the protein. Exon I contains domains A and B, while exon 12 contains almost all of domain D, and exon 13 contains the alternatively spliced domains E and F. Domain C, the central homologous domain implicated in the binding of synapsin I to actin and to synaptic vesicles, is divided into nine exons.