Modifications of spermatogenesis and anatomy of male reproductive tracts caused by radiation (fast electrons) are described in Acarus siro and compared with data from other arthropod taxa. Radiation induced severe changes in gonial cells, including dilatation of endoplasmic reticulum and disruption of mitochondrial cristae. The latter sometimes created roundish myelin structures inside the mitochondrial matrix. Damage in developing cells caused an interruption of spermatogenesis and reduced the number of spermatids produced; high doses can cause sterility.