PROC. ENTOMOL. SOC. WASH. 84(4). 1982. pp. 729-741 THE MAYFLIES OF NORTHEASTERN MINNESOTA (EPHEMEROPTERA)' T. M. Lager, M. D. Johnson, and W. P. McCafferty (TML) 636 State Street, Menasha, Wisconsin 54952; (MDJ) P.O. Box 368, Teton Village, Wyoming 83025; (WPM) Department of Entomology, Purdue University, West Lafayette, Indiana 47907. Abstract. — Analysis of the Ephemeroptora of northeastern Minnesota is based primarily on extensive sampling of larvae from St. Louis and Lake counties. Of 54 identifiable and nominal species. 33 represent new state records. Several notable range extensions are documented. The fauna has a strong affinity with that of eastern Canada; other faunistic relationships are also present. In Minnesota, Baetis macani hundyae Lehmkuhl larvae inhabit streams and complete their life cycle one and a half months earlier than larvae in the northern extreme of their range where they inhabit tundra ponds; larvae and adults are similar in size throughout their range. The Ephemeroptera of Minnesota, especially in the northeastern region, are largely unknown. Although Daggy surveyed the mayfly fauna throughout the state (Daggy, 1938, 1941), he published only a small part of the data (Daggy, 1945). Thomas Say, who accompanied an expedition on the Min-nesota and Rainy rivers in the 1820's, was probably the first to report on the state's mayflies (Say, 1823). Burks (1953) documented Minnesota dis-tributions for species found in Illinois. Additional records occur in system-atic works (Banks, 1910; Needham et al., 1935; Lewis. 1974; McCafferty, 1975; Edmunds et al., 1976; Berner, 1978; Bednarik and McCafferty. 1979; Morihara and McCafferty, 1979b; Pescador and Berner. in press) and in ecological studies of certain species (Waters, 1966; Fremling, 1973; Waters and Crawford, 1973; Hall et al., 1975). In this paper we document the mayflies of northeastern Minnesota, pre-dict those species likely to occur in the northern region of the state, show faunal relationships to other areas of North America, and discuss the effi-cacy of factors which may influence the faunal distributions. Larvae were ' Part of the Minnesota Regional Copper-Nickel Study, Environmental Quality Board. St. Paul. Minnesota.