One of the two most speciose genera in the Mniaceae, Rhizomnium includes 14 to 16 named taxa with the likelihood that more remain to be described. Our previous research established that R. pseudopunctatum is an allodiploid whose haploid progenitors are R. gracile and R. magnifolium. In the course of that work, we sampled five additional species as possible progenitors. Using isozymes from starch-gel electrophoresis, we screened all eight species to compare levels of genetic variation within species, genetic distances between species, and genetic population structure. Levels of variation are generally lower than for species of Plagiomnium but are similar to those for species of Cinclidium. Relationships of species based on genetic data are mostly congruent with Koponen's (1973) “assumed phylogeny” of Rhizomnium, but there are also some clear conflicts. For example, R. appalachianum is highly divergent from the other three species placed in sect. Macromnium, and R. glabrescens does not seem to fit in sect. Rhizomnium. Genetic distance data, however, do support Koponen & Sun's (2016) recognition of R. chlorophyllosum as a separate species from R. punctatum.