The effects of inter- and intraspecific interference on size hierarchies (size inequalities) were investigated in populations of Trifolium incarnatum and Lolium multiflorum. 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 equality. Results support a model of plant interference in which large plants are able to upsurp resources and suppress the growth of smaller individuals more than they themselves are suppressed. While interference decreases mean plant mass, it increases both the relative variation in plant mass and the concentration of mass within a small fraction of the population. -from Author