Observed regularities in the size composition of pelagic ecosystems are explained by a theoretical model based on trophic processes. The model assumes, in accord with available evidence, that prey and predator sizes are simply related, and that growth and metabolism satisfy similar functions of body (particle) size when averaged over entire trophic levels. The model appears suited to other community types that satisfy these conditions, and suggests that trophic processes are complementary to competitive displacement in determining that particle density is a linear function of the logarithm of particle size.