The structure of early, intermediate and late spermatids as well as of the spermatozoon of the mite Erythraeus phaiangoides is described. The acrosome and the attached acrosomal filament are formed during early spermiogenesis. The nuclear material, undergoing gradual condensation, also adheres to the acrosome. At the same time, the nucleus strongly elongates and the nuclear envelope almost completely disappears. Peripherally distributed, flattened and tubular cisternae appear in the late spermatid. These cisternae develop as invaginations of the plasmalemma and their membranes communicate with the plasma membrane. Immediately before entering the lumen of the testis, the spermatozoa develop an additional outer membrane formed on the outside of the plasmalemma. During spermiogenesis mitochondria undergo a structural reorganization, with electron-dense granules appearing in their matrix