Stephanodiscus tenuis Hust., Melosira islandica O. Müll., and Diatoma elongatum (Lyngb.) Ag., on a cell volume basis, were the three most important species from January 1964 to July 1965 in the surface water of Lake Ontario near Gibraltar Point.Stephanodiscus tenuis was by far the most abundant species. It was present during most of the year and accounted for about 87% of the total number of algae present during the spring outbursts.In more central areas of the lake Melosira islandica and Asterionella formosa Hass. replaced Stephanodiscus tenuis as the most important species.On the basis of the major phytoplankton species present the central area of Lake Ontario appears to be oligotrophic. High numbers of Stephanodiscus tenuis in inshore waters may be indicative of a more eutrophic environment.