A vigorous male Drosophila melanogaster should produce from 10, 000 to 14, 000 offspring; a smaller, less vigorous male should produce about 7, 500. The average fertile period of males is about 32 days. Following the fertile period is a sterile period which varied in time in the males used from 21 to 34 days. Sperm were observed in the testes of all males after their fertile period was over. Mitotic figures were found in sections of the testes of males after they had come to the period of sterility, showing that sperm were still being formed. Tests were made to find out whether the failure of sperm from old males might not be due to a deficiency of some fluid from the accessory glands. The results indicated that this was not the case. No sperm were found in the sperm receptacles of females that had mated to these males during their sterile period, excepting, of course, the cases of double matings. Therefore, the impotency of the old males must be due to a loss of function in the ejaculatory apparatus.