Several testicular and spermatological parameters of 48 male channel catfish, Ictalurus punctatus (Rafinesque) were studied during 6 months (November through April) outside the spawning season. Two stocks (a domestic and a wild one) and within each stock 2- and 3-year-old fish were compared. Within age-groups, the domestic fish were considerably larger than the wild ones. In each stock, the 3-year-olds outgrew the 2-year-olds. The testes consisted of a white spermatogenic and pink glandular part, averaging 1.14 and 0.56 g respectively. A significant increase in both parts was noticed in March, continuing during April. One 3-year-old male in each stock was azoospermic, even though these fish had well developed testes and exhibited secondary sexual characteristics. Viable sperm was present in every month at an average rate of 73.2% of the spermatozoa. The majority was in progressive motion with some vibrating in loco. The gonosomatic index averaged only 0.22%, and reached a peak value of 0.32% in April. The mean gonadal sperm concentration was 5.5 × 109spermatozoa/g wet testicular tissue and a maximum of 7.4 × 109occurred in April. Both stocks produced a similar number of spermatozoa/g. As a group, however, the testes of the older fish contained twice as many gametes per g as those of the 2-year-olds. A highly significant (P < 0.01) correlation of 0.51 existed between the width of the head (a male secondary sexual characteristic) and the gonadal sperm concentration. No pronounced differences between domestic and wild stocks were observed for most variables, but some size and/or age effects occurred.