Relative growth rates of three freshwater plankton diatoms-Asterionella formosa, Fragilaria crotonensis, and Tabellaria flocculosa var. asterionelloides-are described from cultures suspended at various depths and during several seasons in the lake Windermere. Seasonal variation in rates recorded near the surface (i m. depth) is interpreted in terms of seasonal changes in temperature and daylength. Rates recorded for Asterionella and Fragilaria are generally similar, but are approximately twice the rates obtained with Tabellaria. Depth profiles of relative growth rates are of similar form in all species, and normally show lightsaturation near the surface. The shape of profiles for Asterionella is in good agreement with estimates of photosynthesis integrated over the growth periods. The parallelism between photosynthetic and relative growth rates of Asterionella is further illustrated from laboratory experiments: an approximate interconversion, under certain conditions, is given.