Cooperative behavior has been observed in cetacean species in a variety of situations, including foraging, mate acquisition, play, and epimeletic behavior. However, it has proven difficult to demonstrate cooperative behavior among dolphins in more controlled settings. Dolphins (Tursiops truncatus) in this study were exposed to a task that could most easily be solved if dolphins cooperated. Six dolphins were provided opportunities to solve the task and had to learn to do so without human intervention or training. Two adult males consistently and spontaneously jointly interacted in order to most efficiently open a container that contained fish by pulling on ropes at the ends of the container. Their interaction was viewed as cooperative because each dolphin pulled on their respective ropes in the opposite direction, which resulted in one end of the container opening. The dolphins did not show aggression toward one another while solving the task, and both dolphins consumed the food after the container was opened. They also engaged in synchronous non-aggressive behaviors with the container after the food had been consumed. It is possible that some of the remaining four dolphins would have cooperated, but the two successful dolphins were dominant males and their interest in the apparatus appeared to preclude other animals from participating.