The male genital apparatus of the Mediterranean teleost, Zoosterisessor ophiocephalus, consisting of pairs of testes, spermatic ducts and seminal vesicles are described during mature (spawning season) and immature (interspawning season) stages. Seasonal changes in the testes and in male GSI (gonadosomatic index) and SVSI (seminal-vesicel-somatic index) were studied during the course of one year. Thereby it turned out that during early prespawning season the testis and the spermatic duct contain mature spermatozoa already. Environmental factors of the study area and the living space of Z. ophiocephalus are described. The function of the seminal vesicle is discussed.
