Spermatogenesis and sperm ultrastructure of the trombiculid mite Hirsutiella zachvatkini (Schluger 1948) have been investigated using transmission electron microscopy and compared with other arachnids studied. Sperm differentiation takes place in groups of synchronously developed germ cells of the two large sac-like paired testes. Each testis is composed of a secretory epithelium, which occupies their medio-ventral regions, and of a germinative epithelium situated in the latero-dorsal parts of testes together with large somatic cells. The germ cells are represented on sections by spermatogonia, spermatocytes, early, middle and late spermatids, and mature spermatozoa. Spermatocytes and spermatids contain two centrioles, which disappear afterwards, and a small Golgi-like structure forming an acrosomal cistema. Mature spermatozoa, which lie both within the meshes of somatic cells and also free in the lumen of testes, are compact oval aflagellate cells provided with peripheral channels. They also contain an acrosome, flattened between the cell membrane and the round electron-dense chromatin body, an oval body of lesser density lying in close proximity to the chromatin body, and a group of 5–7 mitochondria with spherically arranged cristae situated immediately behind the nuclear bodies. An acrosomal filament may be sometimes seen beneath the acrosome in the middle spermatids and disappears in the mature spermatozoa. These findings show that the mode of differentiation and pattern of organization of the male sex cells in trombiculid mites are of rather primitive type compared with other acarine spermatozoa.