The primary ectoparasites of beaver (Castor canadensis), Muskrat (Ondatra zibethicus), Round-tailed muskrat (Neofiber alleni), and the introduced nutria, Myocastor coypus, are tiny listrophoroid mites that clasp individual hairs and apparently dip their mouthparts into hair follicles for nourishment. The tiny size and hair-clasping habit appears to help them remain attached while their host is in water. The beaver hosts a large number of species of the genus Schizocarpus (Chirodiscidae); the species are of four different groups, each of which occurs on different parts of the beaver. Moreover, different species occur on beavers in different parts of their range. This has been referred to as multispeciation. Six species of Listrophorus (Listrophoridae) are known from muskrat, but usually five parasitize hosts throughout most of their range. Three listrophorid species occur on the round-tailed muskrat; two Listrophorus and one Prolistrophorus. Surprisingly, in view of the other three aquatic rodents examined, nutria have only one species of parasite of this type, Myocastoroides myocastor (Atopomelidae). These mites are in similar habitats (the fur of aquatic rodents), but are of different evolutionary origin. They appear to have converged evolutionarily in terms of size, shape, habitat and habits.