The incidence of abnormal spermatozoa in normal and dichlorvos-treated Armigeres subalbatus was examined morphologically by electron microscopy. The mosquitos used in this study had been maintained in the laboratory for 160 generations after being collected in Kanagawa prefecture. For control mosquitos, ten seminal vesicles were removed from male adults 3days after emergence, and sectioned at the middle. The numbers of all normal and abnormal spermatozoa in one side of the double-lumen of a cross-section were counted except for head nucleus parts, tail ends, and tangential sections which could not be identified as normal or abnormal. Normal spermatozoa had 1 flagellum and 2 mitochondrial cords, but abnormal ones had unusual numbers of them. Control mosquitos showed a 4,37% incidence of abnormal spermatozoa as averaged from data on 10 seminal vesicles. For the insecticide treatment, living 4-th stage larvae which were exposed to 0.201ppm of dichlorvos (DDVP) for 24 hours were put back into normal water without DDVP. The seminal vesicles from 10 male adults 3days after emergence were examined likewise to control ones. The count was taken from both sides of the double-lumen of a cross-section. The average incidence of abnormal spermatozoa treated with DDVP was 19.1% which was 4.4 times higher than in control mosquitos. The total number of spermatozoa decreased in vesicles which were treated with DDVP. A large number of abnormal spermatozoa consisted of 1 flagellum and 1 or 3 mitochondria. The wall of seminal vesicles treated with DDVP became thicker than that of controls, and contained many vacuoles in the cytoplasm.