Ivlev's model of competition for food is evaluated. This model expresses ration size as a function of prey density and population size. Ivlev's model is elaborated upon by including a term concerning the size (weight) of the animal. Alternative ways in which the effect of population size upon intraspecific competition may be described are suggested. Ivlev's model and two alternatives are fit to data from an amphipod population by nonlinear regression. Results suggest that intraspecific competition affects the efficiency with which the consumer captures its food, rather than affecting the maximum amount it is capable of eating. This approach is generalized to include both intra- and interspecific competition for food.