The accessory gland of male insects is a secretory tissue of the genital tract made up of several distinct cell types. It secretes components of the ejaculatory fluid which have an important effect on the postmating behavior of the female. We have examined the sequence, structure, and expression of a gene, mst 316, expressed exclusively in the accessory glands of male Drosophila melanogaster. The mst 316 RNA encodes a small, basic protein of 52 amino acids that exhibits features common to precursors of secreted peptides, including a hydrophobic N-terminus. The tissue-specific expression of the mst 316 gene was studied using an mst 316--lacZ hybrid gene inserted into Drosophila by germ line transformation. The mst 316-lacZ fusion protein is expressed exclusively in the "main" cells of the accessory gland. It is first detected upon eclosion and exhibits a burst of synthesis in the first 3 days of adult life. The synthesis of the fusion protein is stimulated by mating, so that beta-galactosidase activity levels are two- to sixfold higher in males allowed to copulate with females compared to virgin male controls of the same age. The mating-stimulated synthesis of the mst 316-lacZ fusion protein, and by inference of the native gene product, appears to be due at least in part to increased transcript levels.