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The effects of inter— and intraspecific interference on size hierarchies (size inequalities) were investigated in populations of the annual plants Trifolium incarnatum and Lolium multiflorum. Variables experimentally manipulated included plant density, species proportions, soil fertility, and spatial pattern of plantings. Densities were below those for extensive density—dependent mortality. Size inequality always increased with increasing density. Plants grown individually showed very low inequality, while plants grown at the highest density had the most developed hierarchies. Size inequality usually increased with an increase in productivity when interference was occurring. When dominant in mixtures, Lolium showed less size inequality than in monoculture, while the suppressed species, Trifolium, usually displayed an increase in inequality. Spatial pattern appeared to be less important than other factors in causing size inequalities; plants sown in a uniform spatial pattern showed significantly lower size inequality than plants sown in a random pattern in only one out of four cases. Inequality in reproductive output of Trifolium, as estimated by dry mass of flower heads, was always greater than inequality in plant in dry mass. The results support a model of plant interference in which large plants are able to usurp resources and suppress the growth of smaller individuals more than they themselves are suppressed. While interference decreases mean plant mass, it increases both relative variation in plant mass and the concentration of mass within a small fraction of the population.