The degree of monopolization of resources is thought to be higher in groups that compete by interference than by exploitation. However, the monopolization of resources will presumably depend on (1) whether the dispersion of resources is economically defendable, and (2) whether some competitors have the ability to defend these resource distributions and hence capitalize on this potential. We tested for an interaction between the effects of temporal resource dispersion and aggressiveness on the degree of resource monopolization in a foraging system. Two species of fish differing markedly in aggressiveness (high: convict cichlids, Archocentrus nigrofasciatus; low: goldfish, Carrasius auratus) were allowed to compete intra-specifically in groups of four for food that was either potentially defendable (arrived asynchronously) or not (arrived synchronously). As predicted, the monopolization of food, measured as the coefficient of variation of food eaten within groups, was significantly higher in the defendable than in the undefendable treatment for convict cichlids but not for goldfish. However, the monopolization of food was higher in the non-aggressive goldfish than in the aggressive convict cichlids. Future studies should quantify and compare the monopolization in species that compete primarily via scramble competition to those that use primarily resource defence.