Testes ultrastructure and spermatogenesis were studied in two species of Strepsiptera (Insecta), namely, Elenchus tenuicornis and E. japonicus, using light and electron microscopy. In both species, the testis is paired and consists of several large irregularly shaped follicles. Each follicle consists of a single clone of germ cells surrounded by a thin epithelium. During the larval and pupal stages, all the germ cells of each testis develop synchronously, and at eclosion, the gonads contain solely mature sperm. One of the most interesting findings is the morphogenesis of a large nuclear vesicle bounded by the fenestrate parr of the nuclear envelope. This vesicle contains an electron-dense spherical structure, the chromatoid body. At the end of spermiogenesis, both the nuclear vesicle and its chromatoid body are eliminated with the excess cytoplasm. Large drops of residual cytoplasm containing several nuclear vesicles are present in the lumen of the testis and inside the cytoplasm of phagocytic cells. (C) 1998 Elsevier Science Ltd. All rights reserved.