Winterkill of fish in southern Michigan lakes during 1944–45 provided an opportunity to study the effect of different degrees of oxygen depletion on the extent of mortality and survival of several common species of fishes. Eleven lakes were investigated. Several inches of snow cover remained on the lakes from December 11 to February 15. This abnormal persistence of snow cover was accompanied by oxygen depletion in some of the lakes to mid-winter lows occurring about February 14. The maximum concentration of oxygen in vertical series taken at about the time of greatest depletion ranged from 0.2 to above 5.0 p.p.m. in the different lakes. Where oxygen depletion was severe this maximum occurred only in a thin stratum immediately beneath the ice. Heavy winterkills occurred only in those lakes in which the dissolved oxygen was depleted to approximately 0.6 p.p.m. or less. The mortality was greatest among bluegills and largemouth bass, apparently 100 percent in some lakes. Among the pumpkinseeds, mud pickerel, northern pike, chub suckers, bullheads, and golden shiners there was a large rate of survival even in lakes where the oxygen was reduced to 0.3 or 0.2 p.p.m. No complete kill of all fish in a lake was encountered.