POLYPLOIDY is relatively scarce in the animal kingdom, and is apparently almost entirely confined to those species in which reproduction is partheno-genetic, hermaphroditic or clonal1. Such animals usually do not have a heterogametous sex-determining mechanism to be disturbed by polyploidy. The hermaphroditic nature of freshwater pulmonate snails and their ability for self-fertilization2 suggest the possibility that there might be a considerable degree of polyploidy among the large number of species of this group because a single accidental polyploid individual could give rise by self-fertilization to a polyploid strain, rather than producing only sterile triploids, if cross-fertilization were obligatory3. So far there has been little reliable evidence for polyploidy in aquatic pulmonate snails. (An earlier indication of polyploidy, based on the chromosome numbers reported for Physa gyrina 4, and P. acuta (= integra?)5 n = 6 and 18, respectively, is due to erroneous observations concerning P. gyrina). Six specimens of this species from several localities in Colorado from which animals were taken for the earlier investigation were found to have haploid chromosome numbers of eighteen (J. B. Burch, unpublished work). The high chromosome number (n = c. 59) reported for the limpet Ancylastrum costulatum (= Ancylus fluviatilus Muller?) was not considered to be polyploid, but due to diphylogeny in freshwater snails6.