Thirty-five species of digenetic trematodes, representing 25 genera and 8 families, have been studied with regard to chromosome number and morphology. Four of the species belong to the family Allocreadiidae, 1 to the Clinostomatidae, 2 to the Lecithodendriidae, 1 to the Cephalogonimidae, 2 to the Gorgoderidae, 6 to the Plagiorchiidae and 18 to the Reniferidae. For each species the chromosome number was found to be constant, although variations occur within families and genera. Chromosome numbers of 7, 8, 9, 10, 11 and 14 are reported for species in the various families. Evidence, based on cytological characters, does not support Faust and Dollfus in placing the Allocreadiidae in the same superfamily with the Plagiorchiidae but it strengthens McMullen's suggestion that the family Lecithodendriidae shows relationship to the families Plagiorchiidae and Reniferidae and should be included in the same superfamily with them. The progressive increase in chromosome number suggests aneuploidy and it is possible tha...