Phenotypic polymorphism is common in animals, and the maintenance of multiple phenotypes in a population requires forces that act against homogenizing drift and selection. Male-male competition can contribute to the stability of a polymorphism when males compete primarily with males of the same phenotype. In and around a contact zone between red and blue lineages of the poison frog Oophaga pumilio, we used simulated territorial intrusions to test the non-exclusive predictions that males would direct more aggression toward males of (i) their own phenotype and/or (ii) the phenotype that is most common in their population. Males in the monomorphic red and blue populations that flank the contact zone were more aggressive toward simulated intruders that matched the local coloration. However, males in the two polymorphic populations biased aggression toward neither their own color nor the color most common in their population. In sympatry, the rarer color morph gains no advantage via reduced male-male aggression from territorial males in these O. pumilio populations, and so male aggression seems unlikely to stabilize color polymorphism on its own. More broadly, these results suggest that the potential for divergent male aggression biases to maintain phenotypic diversity depends on the mechanism(s) that generate the biases and the degree to which these mechanisms persist in sympatry. This article is protected by copyright. All rights reserved.