Mixed-species associations are temporary aggregations of individuals of different species involved in similar activities. Such associations form for foraging, protection against predators, and social advantage. Mixed-species groups in delphinids are frequent in the wild. We aimed to understand the ecological significance of mixed-species group formation by 2 tropical delphinids, the spinner dolphin (Stenella longirostris) and the pantropical spotted dolphin (Stenella attenuata), in waters surrounding the island of Mayotte in the southwestern Indian Ocean. We used sighting data collected year-round from 2004 to 2009. We encountered a total of 67 mixed-species groups (comprising 21% of all groups observed) of spinner and pantropical spotted dolphins around Mayotte. No daily or seasonal variability in the occurrence of associations was detected. Behavioral activities of single- and mixed-species groups differed significantly. Foraging was observed only in single-species groups of pantropical spotted dolphins. Mixed-species groups were larger than single-species groups. When in association, spinner dolphins used deeper waters than while in single-species groups. No evidence of association for social advantage was observed. We suggest that spinner dolphins associate with spotted dolphins for protection against predators when transiting between resting areas.