The pattern of spermatogenesis was studied during an annual reprodutive cycle in adult male brown trout. The part of the testicular volume occupied by the nuclei of germ cells and of Sertoli cells, the lobular lumen and the extra-lobular space were measured. The number of germ cells was estimated by the total volume occupied by each cell divided by the individual volume of each cell. The permanent number of spermatogonia A (spermatogonia isolated or in groups of 2) was about 50 million. Each GA cell produced several spermatogenetic waves during the same reprodutive cycle. Sperm production was about 25 X 10(9)/g of testis/year, or about 6.8 X 10(7)/g of body weight/year. Males stocked separately from females, thus presumably not releasing spermatozoa, were killed in December, January and February. Since these fish contained the same amount of spermatozoa it appears that the process of spermiation initiated in December did not continue in January and February. During this period, some spermatozoa were phagocytized by macrophages (spermatophages) in the lobular lumen and by Sertoli cells.