Twenty-six species of Tubificidae, 12 Naididae, and 1 Lumbri-culidae were found in 3 regions of Lake Michigan[long dash]Green Bay, the southern end of the lake proper, and the harbor at Ludington, Michigan. One new naidid species [Piguetiell michicanensis] is described. Methods of identification of some species are discussed and illustrated. The abundance of oligochaetes and distribution of certain species vary with depth and location. Data are presented on the effects of the environment on the distribution of certain species. The presence or absence of some species reflected the quality of the environment; some were prevalent in polluted waters whereas others were restricted or absent. Possible changes in composition and abundance of species are outlined if organic enrichment increases in Lake Michigan.