Synaptonemal complexes were demonstrated in 7 microsporidian species [Baculea daphniae, Glugea habrodesmi, Gurleya chironomi, Nosema orthocladii, Pleistophora simulii, Theolhania sp., T. fibrata and Tuzetia debaisieux]. It is assumed that meiosis and consequently karyogamy occur during the life cycle. Meiosis occurs at the beginning of sporogony; karyogamy occurs between the spore stage and the end of merogony, i.e., during the poorly known part of the life cycle. In the species studied with uninucleate spores and diplokaryotic merogony (i.e., Thelohania), the 2 joined nuclei (each of which contained meiotic chromosomes) not only failed to fuse but actually separated at the beginning of sporogony; afterwards, each nucleus underwent meiosis. Their separation is accompanied by the appearance of an organelle whose structure and function is poorly understood but which resembles the kinetic center. The Nosema spp. studied did not have synaptonemal complexes and their life cycles are consequently difficult to understand. Karyogamy and meiosis may occur during the unobserved part of the life cycle or sexual phenomena may be absent altogether. In the latter case, the Nosema-type life cycle might be limited to vegetative multiplication which could be explained by the dimorphism theory of microsporidia. The microsporidian life cycle apparently does not involve haploid organisms contrary to the sporozoan life cycle.