The vas deferens of Penaeus setiferus was divided into 4 anatomical regions (1A, 1B, 2A, 2B, 3, and 4), and the functional morphology of the first 3 regions was evaluated using microscopical techniques. Segments 1A and 1B are small collecting tubules comprised of a low columnar epithelium surrounded by a thin muscle coat and connective tissue that is rich in collagen. These tubules transport sperm from the testes to segment 2A, and their epithelium appears to secrete part of the sperm-supporting matrix. Segment 2A is much thicker and is comprised proximally of a blind pouch where sperm are concentrated and a longer region containing 2 lumina. One lumen (1) is occupied by the sperm mass and the surrounding epithelium appears to secrete the primary spermatophore layer. The other lumen (2) is smaller in diameter in segment 2A and contains 2 acellular materials. Segment 2B is similar to 2A except that the epithelial layers separating the 2 lumina become an incomplete partition and a new coat, the secondary spermatophore layer, is formed outside the primary spermatophore layer. The relative size of the 2 lumina becomes reversed in segment 2B, and the leading end of the developing spermatophore appears to be sealed by secretory product in the distal region of 2B. Segment 3 is extremely thin but is surrounded by numerous muscle cells which undergo smooth rhythmic contractions in vitro. These contractions move material stored in segment 2B toward the ampoule (segment 4). The epithelium of segment 3 releases electron-dense material into the lumen by exocytosis and also undergoes extensive apical blebbing. These data suggest that the penaeid vas deferens functions in production of the sperm-supporting matrix, consolidation and orientation of sperm, secretion of some of the acellular components of the spermatophore, and storage of the developing spermatophore.