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Competition at different levels of phosphorus availability was observed in continuous cultures of freshwater phytoplankton communities. Although dilution rates ranged 10‐fold in all, the outcome of competition was usually similar among cultures and resulted in dominance of the small diatom Synedra acus. Species growth rates decreased significantly with increasing cell, or colony, size among the 16 species examined. A variable internal stores model of algal growth, combined with functions relating its species‐specific parameters to cell size, correctly predicted the observed inverse correlation between cell size and competitive ability. The model’s predictions, and the empirical correlation between size and growth, were poor at low dilution rates (<0.2–0.3 d⁻¹), probably due to cell death. The results indicated that variation of phosphorus supply is unlikely to be a major selective influence on the size or species …