The genus Arrenurus is by far the largest of the 50 genera of Hydracarina known to occur in Michigan, both in number of indivduals as well as in the number of species. Of the more than 26,000 water mites collected during the summers of 1949-1952, approximately 8,500 were arrenuri. The number of species and varieties of this genus known from the state is 63, three of which are based upon literature records only. Some idea of the richness of this fauna may be obtained by a comparison with the number of species recorded from other areas. Lavers (1945) listed 30 species and varieties as occurring in the State of Washington. Marshall (1940) recorded 40 species and varieties from Wisconsin. From Europe, where collecting has been much more extensive than i nthis country, the species’ lists are longer. Viets (1936) stated that 69 different arrenuri have been found in Germany, while Soar and Williamson (1929) listed 52 for the British Isles. Lundblad (1944) recorded 53 species and varieties from south Brazil and Paraguay. The genus Arrenurus is world-wide in distribution, and one of the most abundant groups. Viets (1936) stated that there were approximately 375 described species of the genus. The number of described species has grown considerably since that time and now probably stands close to 500. The first North American arrenuri were described by Koenike (1895) from material sent to him from British Columbia and Alberta. Koenike described four species which, though poorly figured, are now recognizable, due largely to recollecting at the original localities by the late Charles Lavers. It was not until 1903 that other arrenuri were reported from North America. Ruth Marshall (1903) published on 10 species of the subgenus Megaluracarus, five of which she described as new, and five which she considered to be identical with European species. Piersig (1904) recognized the specific distinctness of the latter five species from Marshall’s figures, and gave them new names. Piersig (1905) again described as new, a species which Marshall (1904) listed as a European form. The largest single publication on North American arrenuri is the work of Marshall (1908) listing 46 species, 36 of which were described for the first time (later, two of these species were found to be synonymous with Koenike’s species). Marshall published smaller papers on the group until 1944. Only two other authors have published on new species of North American anneruri. Lavers (1945) described 11 new species from the State of Washington and Munchberg (1951) reported a new Arrenurus from Kentucky. Prior to this study there were 78 species and varieties belonging to this genus known from North America. Ten more are described here which brings the total to 88. Distribution information on the genus Arrenurus within Michigan is based on the results of four years of collecting by the writer, literature records from Marshall (1908, 1914, 1927), and unpublished data belonging to Karl F. Lagler. Further collecting will probably change the known distribution pattern of some of the species. However, a rather large percentage of the arrenuri are probably limited in their geographic distribution within the state. In some cases this spatial limitation may be due to absence of certain habitat types. Extensive collecting in Cheboygan County, where habitats similar to those of southern Michigan are found, failed to show many of the species found in the latter area. In Michigan, arrenuri were taken in all types of standing water but never in springs or swift streams. The genus does, however, occur in the latter habitats in other areas. Lundblad (1944) listed 19 species from South America which were collected in streams, three of which were taken exclusively in this habitat. However, no mention was made of the speed of the streams. A. fontinalis Viets is a European species found in springs.