The autumnal and over-winter limnological characteristics of two small eutrophic lakes differed considerably from those of a third eutrophic lake; all were close to each other in the southwest interior of British Columbia. Thermal and chemical stratification was well defined in Marquette and Corbett lakes because local topography provided protection from wind action; stratification in the more exposed Courtney Lake was ill defined. Average wind velocities during summer and autumn on Courtney Lake were 4.3 times as great as, and more unidirectional than, those on Corbett Lake and probably Marquette. Complete natural autumnal oxygenation and circulation in Courtney Lake provided comparatively high concentrations of dissolved oxygen which permitted the over-winter survival ofSalmo gairdneriandRichardsonius balteatus. Although Marquette and Corbett lakes were isothermal in late autumn, oxygen concentrations were well below saturation levels and circulation was probably incomplete. Consequently oxygen depletion during winter was severe andS.gairdneriandSalvelinus fontinaliswere subject to over-winter mortality.Experimental circulation of Corbett Lake, just prior to ice cover, confirmed the hypothesis that incomplete autumnal oxygenation is a cause of "winter kill" of fishes. Artificial autumnal circulation of the lake provided an oxygen concentration sufficient to prevent overwinter fish mortality. However, over-winter mortality of the entire fish population did occur in the control lake, Marquette (incompleteautumnal oxygenation) but not in Courtney Lake (completeautumnal oxygenation).