Dolphin communicative signals show great plasticity. Dolphins modify signal structure to cope with their environment, in response to stress, and in some species to mimic group members. Hence, whistle structure variations may offer insights to interspecific associations among dolphin species, which although temporal and opportunistic are common. In this study, I test the hypothesis that interspecific interactions influence dolphin whistle structure, particularly during social events. The study took place in the Southern Caribbean coast of Costa Rica, where interspecific associations of the distantly related Guyana and Bottlenose dolphins occur on daily basis. The results indicate that interspecific groups emit whistles that show intermediate whistle structure compared to whistles emitted in intraspecific groups. This pattern is seen during social interactions between species, but not when interspecific groups are traveling. Social events in interspecific groups were of antagonistic nature, where Bottlenose dolphins isolated and harassed one or two Guyana dolphins. Contour data suggest that the most vocal species during these encounters was the Guyana dolphin. Therefore, the observed modifications in whistles structure likely reflect a stress response by the Guyana dolphins. Another alternative explanation includes signal convergence between interacting species. However, to understand the nature of these potential modifications, future studies should combine acoustic tags and directional recording systems to follow the vocalizing animals. Despite the shortcomings of this study, it provides some of the first insights into dolphin interspecific communication, providing evidence of overall signal change during interspecific interactions.