The temporal dynamics of phytoplankton were examined in The Loch, a subalpine lake in Rocky Mountain National Park, over the winter seasons of 1987-88 and 1988-89. The Loch was ice-covered from early November until early to mid May. The pattern of phytoplankton biovolume during ice-cover was consistent between the two years with maxima occurring in November/December and February/March. This pattern resulted principally from the contribution of Asterionella formosa Hass. Other dominant phytoplankton species in terms of biomass (Dinobryon sertularia Ehrenb., Cryptomonas ovata Ehrenb., and Peridinium cinctum (Müll.) Ehrenb.) collectively contributed from 10 to 90 % of the total cells. Algal composition changed throughout the winter and individual species varied in abundance with depth. The same dominant (and most of the rare) taxa were present both years. They varied in time of occurrence and abundance, but did not occur at the same time in both years. Phytoplankton species composition continually fluctuated throughout the winter. Because of the stability afforded by ice-cover, algal species succession was not driven by thermal regime or by wind induced changes in the mixed depth. Nor did grazing by the winter zooplankton assemblage, composed nearly exclusively of cyclopoid copepods and rotifers, adequately explain the phytoplankton dynamics. Freeze concentration of water (concentration as ions are excluded in the formation of ice) in early winter may be responsible for the early phytoplankton bloom.