The effect of repeated microwave irradiation (2375 MHz, CW) on mutagenic changes in Drosophila melanogaster was investigated. Oregon-R males were exposed to sublethal doses of microwaves (15 W/cm2 for 60 min, 20 W/cm2 for 10 min, and 25 W/cm2 for 5 min) for 5 days. The Muller-5 cross was used to detect sex-linked recessive lethal mutations. 4 lethals were found in treated groups but their frequency was not significantly different from that of the control group. No cumulative effect of repeated exposures on the mortality of the treated males was observed; on the contrary, their mortality decreased with the number of exposures. Irradiation did not affect the sex ratio of the F1. A significant decrease in the number of F1 offspring was noted in the group exposed to the power density of 15 W/cm2. A negative thermal effect of microwaves on male germ cells was probably manifested by this long exposure.