Pachytene idiograms are presented for males and females of Schistosomatium douthitti, Ornithobilharzia canaliculata, and Schistosoma mansoni. Although the heteromorphic sex chromosome pairs of S. douthitti and O. canaliculata are similar in morphology at mitosis, their pachytene configurations show that they differ in the arrangement of pairing and differential segments. S. mansoni lacks a heteromorphic pair of sex chromosomes at mitosis, but at pachytene in females the longest bivalent, corresponding to the XY pair of the other two species, is composed of one euchromatic and one partially heterochromatic chromosome. Since in males the corresponding bivalent is composed of two euchromatic chromosomes, it is concluded that the partially heterochromatic chromosome is a Y and that females of S. mansoni are heterogametic, as are those of the other two species.