Abstract Anopheles mosquitoes are important candidates for genetic control strategies. However, little is known about sperm quality and quantity as determinants of male reproductive success. In this study, sperm quantity and length variation were assessed in testes of un-irradiated and irradiated Anopheles arabiensis. Male reproductive organs were dissected for sperm and an estimate of the total number of spermatozoa was made. Sperm lengths were measured using imaging software. The effects of irradiation were evaluated for males exposed in the pupal or adult stage to a full (120 Gy) or partially sterilising dose (70 Gy). Sperm length variation in the laboratory strain was compared to the distribution observed in wild males. We also determined the size distribution of sperm lengths in spermathecae of inseminated females compared to those observed in male testes. Sperm quantity increased with age, and 12-day-old males had significantly more sperm in their testes (8214 ± 467) than males aged 3 days (5022 ± 375). Mosquitoes irradiated in the pupal stage had significantly fewer sperm (2982 ± 125) than un-irradiated males (4950 ± 848) although for adult stage irradiation similar amounts of sperm were observed compared to un-irradiated males. Sperm length variation was detected with sperm lengths ranging between <50 and 500 μm. There were no differences in sperm length distribution compared to wild males. Sperm length distributions were similar to those reported for the closely related sibling species An. gambiae s.s. There was no major effect of irradiation on the distribution of sperm lengths in the testes, with the exception that pupal irradiation resulted in a significant increase in sperm numbers in the category of 100–200 μm. Sperm length distributions in spermathecae were different to those measured directly from sperm in the testes and harboured less cells of the smaller (<100–200 μm), and more cells of the larger category (300–400 μm). The finding that testes of pupal irradiated males produce fewer and smaller sperm in comparison to un-irradiated and adult irradiated testes are discussed in the context of genetic control strategies, in particular the Sterile Insect Technique (SIT).