The study of North American Hydracarina has lagged far behind that of European water mites, due mainly to a lack of sufficient workers in this field. What we do know of our hydrachnid fauna is largely the result of research by Ruth Marshall between the years 1903 and 1946. However, around the turn of the century, Koenike and Wolcott, who were the pioneer workers on American representatives of this group, made very significant contributions to our knowledge of a few genera. Studies on the North American hydrachnid fauna have been almost exclusively taxonomic, with very little published on the ecology and life histories of the various species. Even the taxonomic works have been rather restricted in their scope, inasmuch as nearly all collections have been confined to lakes and permanent ponds. This has left the faunas of streams, springs, seepage areas, and temporary ponds almost untouched. Because the species present in Michigan are so numerous, and the amount of work, descriptive and otherwise, yet to be done on them is so great, no attempt has been made to treat all of them in this paper. Rather, the following aspects of the study of the water mite fauna of the state have been treated in detail and it is hoped that they will serve as patterns upon which future investigations may be based: (1) A key to the known genera of the state, with short generic diagnoses and types of habitats occupied; (2) Revision of the Arrenuridae of Michigan; (3) LIfe history of Arrenurus superior Marshall along with notes on water mite life histories in general.