In cichlid fishes, fighting ability, success at nest defense, and fecundity are all positively correlated with body size. Selection should therefore favor the evolution of strategies for the assessment of relative body size during intraspecific interactions. The effect of relative body size on agonistic behavior in the oscar (Astronotus ocellatus) was tested using dummies. As predicted by game theory models, attack was strongly correlated with the size of the dummy relative to the subject. The most intense aggression occurred when dummy size was approximately 75% of subject size. These data support the prediciton that perceived body size asymmetries are important determinants of agonistic behavior in these fishes. Further, the results suggest that oscars can visually assess relative body size, and may estimate relative fighting ability without tests of strength. Subject size was positively correlated with intensity of aggressive behavior. Attack and display frequencies were positively correlated with a subject's body size, while latencies to approach and attack dummies were negatively correlated with subject size.