The male flies of various ages were irradiated with 1150- to 3300-r doses of Co/sup 60/ gamma radiation to elucidate the relative radiosensitivity in different stages of spermatogenesis. This was evaluated by studying the sensitivity pattern for different kinds of mutations, such as visibles, sex- linked lethals, II-III translocations, and induced crossovers in males. A peak in the frequency of recessive sex-linked lethals was observed on day 5 after irradiation, while translocation frequency showed a peak on day 7. The frequency of visibles showed no peak, but the statistical calculation suggested an increase in frequency up to day 5. A period of excessive sterility was observed on days 7 and 8 or days 8 and 9 after irradiation. The first crossovers were observed on day 6 or 7, concurrently with a low fertility but prior to the minimum of fertility. The time required for spermiogenesis should be about 6 to 7 days at 25 deg C in adult males. It was found that the radiosensitivity of fully mature sperms (day 1) was somewhat higher than that of nearly mature sperms (day 2). With further immaturity an increasing sensitivity was obseived for all effects studied up to day 5 or 6.more » However, if sterility is used as a measure of radiation effect, the maximum occurs on days 8 and 9. (TCO)« less