Comparative studies can help identify selective pressures that contributed to species differences in the number and composition of personality domains. Despite being adapted to an aquatic lifestyle and last sharing a common ancestor with primates some 95 million years ago, bottlenose dolphins (Tursiops truncatus) resemble nonhuman primate species in several behavioral and cognitive traits. For example. like chimpanzees (Pan troglodytes), dolphins live in fission-fusion societies, use tools, and have relatively large brains. To determine the extent to which these and other factors contribute to the evolution of dolphin personality, we examined personality structure in 134 bottlenose dolphins. Personality was measured in 49 dolphins using a 42-item questionnaire, and in 85 dolphins using a version of the questionnaire that included 7 additional items. We found four domains. Three-Openness, Sociability, and Disagreeableness-resembled personality domains found in nonhuman primates and other species. The fourth, Directedness, was a blend of high Conscientiousness and low Neuroticism and was unique to dolphins. Unlike other species, but like humans, dolphins did not appear to have a strong Dominance domain. The overlap in personality structure between dolphins and other species suggests that selective pressures, such as those related to group structure, terrestrial lifestyles, morphology, and social learning or tool use are not necessary for particular domains to evolve within a species.