The relative importance of feeding competition in Pan paniscus and Pan troglodytes schweinfurthii is examined in an attempt to understand the major differences in social organization of the two species. P. paniscus at Lomako is characterized by a stronger tendency for association among females than among female P. troglodytes at Gombe. Party size in P. paniscus is dependent on patch size. Feeding competition was more important in small patches than in large patches. The total amount of feeding time by a party in a patch (chimp-minutes) was a measure of patch size that was available for both chimpanzee species. P. paniscus was found to have larger party sizes and to use larger food patches than P. troglodytes. The importance of dispersed ground foods for each species of chimpanzee was compared and, although the results are not conclusive, they indicate that this type of food was equally important in the diets of both populations. Two hypotheses of the ecological basis for differences in social structure are compared in light of this evidence.