Mitotic karyotypes have been analyzed and pictured, and idiograms have been constructed for the following spp. Schistosomatium douthitti (Michigan), Schistosoma mansoni (Puerto Rico, Egypt, Liberia), S. haematobium (W. Africa), S. japonicum (Philippines), Ornithobilharzia canaliculata (Florida), Trichobilharzia physellae (Michigan), T. stagnicolae (Michigan), Austrobilharzia variglandis (Maine) and Gigantobilharzia huronensis (Michigan). Chromosomes were studied in squashes of cercarial embryos of all spp. and also in squashes of adult worms of O. canaliculata. A heteromorphic pair, interpreted as sex chromosomes (as previously reported in S. douthitti females), was identified in O. canaliculata females. No heteromorphic chromosomes were seen in spp. of Schistosoma and Trichobilharzia, although many plates were studied from a number of snails with spp. of each genus. Since material of Gigantobilharzia and Austrobilharzia was from a single snail each, the question of heteromorphic chromosomes in these genera remains unanswered. All spp. studied had 2n = 16 except S. douthitti (2n = 14). Some cercarial embryos of T. stagnicolae had an extra pair of small chromosomes (2n = 18). The karyotypes of S. mansoni from various localities were similar. Karyotype comparisons suggest that study of additional spp. of schistosomes may aid in taxonomic and phylogenetic studies.