Autoimmune responses against testis material were induced in mature male Nile tilapia Oreochromis niloticus, by injecting allogeneic testis homogenate emulsified with Freund's complete adjuvant. Some immunized males decreased their fertility 4 to 8 weeks after the first injection. Sperm motility of these fish decreased markedly. Ultrastructural observations of the testis and proerties of spermatozoa in immunized fish suggested that this reduction was mainly caused by agglutination of spermatozoa by means of anti-sperm autoantibody penetrating the seminal lobules. Eight weeks after injection, globate structures, which seemed to originate from unabsorbed FCA component, occurred throughout the testicular tissue of some fish, possibly resulting in an obstruction of a normal flow of milt in the testis and a failure of milt discharge in immunized fish.