ABSTRACT The effects of colchicine on spermiogenesis in the mouse were examined. Mice were injected intratesticularly with colchicine in concentrations of 10−4M to 10−6M for various periods of time. The ultrastructure of the testes was examined and abnormal sperm in the epididymis were scored. Colchicine degraded microtubules in the testis and the treatment caused abnormalities of the head and acrosome of testicular spermatids and also resulted in an increase on the number of epididymal sperm showing abnormal head structure by 5 days after treatment of the testes. These results suggest that structurally intact microtubules are essential for the normal morphogenetic processes of spermiogenesis.