In an attempt to understand the differential distribution of Ephemera simulans and Hexagenia limbata in intergrading substrates, the author extended his studies of the ecological roles of the laboratory-derived comparisons of respiration, morphology, and behavior. Conclusions drawn, mainly from my earlier and present work and also work reported by Lyman in 1943 and Hunt in 1953, seem to be consistent with field distribution. It appears that H. limbata is not found in streams unless prevailing conditions include undisturbed, fine sediments, for Hexagenia does not burrow into coarse substrates. To the contrary, E. simulans can thrive in gravel and pebble stream substrates in which the oxygen content of interstitial water is greater than 1.20 cc/l. In lakes where Ephemera can burrow into substrates of gravel, marly sand, and marl, it is limited in the fine sediments by the relative inefficiency of its small gills at low oxygen concentrations (<1.20 cc/l). Again, Hexagenia does not occur in coarse substrates. However, in fine sediments such as marl it is found in large numbers and increases in abundance in muds. Most probably its distribution further into lakes is limited by impenetrable peaty substrates or by oxygen stratification (<0.80 cc/l).