Background: Aggressive behaviour can have significant evolutionary consequences - not only within species, but also in the context of heterospecific interactions. Here, we carried out an experimental field study to investigate the importance of phenotypic similarity on levels of aggression between species whilst controlling for familiarity effects using manipulated allopatric stimuli. Specifically, we investigated aggressive responses of territory holding males and females in two species of Neotropical cichlid fish, Amphilophus sagittae and Hypsophrys nicaraguensis, that differ in their phenotypic similarity to our allopatric stimulus species, Amphilophus astorquii. Results: We found that, independent of phenotypic similarity (and correlated phylogenetic proximity) between the territory holders and intruder, territorial aggression was not adjusted in relation to allopatric intruder colour markings that are associated with different levels of threat and known to provoke different responses in a sympatric setting. We also found that males and females did not differ in their overall patterns of aggression adjustment towards intruder cues. Nevertheless, the two focal species, which share the same breeding grounds and external threats, exhibited different sex roles in breeding territory defence. Conclusion: Together with earlier studies assessing hetrospecific aggression in sympatry, our current results highlight the importance of coevolution and learning in species interactions.