Observation was done on the sperm from the testis of the Sugarcane Yellow Stem Borer, Tetramorea schistaceana (Lepidoptera, Olethreutidae). Emphasis was given to the comparison of the ultrastructure of the eupyrene and apyrene, and the effects of y-radiation on the ultrastructure of the sperm in P and F1 progeny of irradiated males.The results showed the number of sperms in most of the sperm bundles was 23, while in some of the apyrene bundles the number was 29. Both the eupyrene and apyrene had an axoneme with a configuration of 9 + 9 + 2 and a couple of mitochondria derivatives, which were more closely associated and uneven in size in the eupyrene while in the apyrene they were separated and similar in size. However, the most obvious difference was that the apyrene lacked elaborate lacinate appendages and reticular appendages, which extended from the plasma membrane of the eupyrene testicular spermatozoa.Irradiation of late pupae at a dosage of 30 KRad resulted in a few flagellar abnormalities in eupyrene sperms and several large bodies with high electron density mixed with the apyrenes in the sperm bundles. Irradiation of p late pupae at a dosage of 15 KRad resulted in a higher frequency of flagellar abnormalities in eupyrene sperms and irregular arrangement and degeneration of sperms in apyrene bundles of testis of F1 males produced from crossing irradiated P males with the untreated females. However, most of the sperms and sperm bundles found in either the treated P males or Fi males were mormal.