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Ultraviolet (UV) coloration varies drastically across species and likely plays fundamentally different roles in facilitating social communication depending on the ecological or social context. While it is well known that UV coloration serves important social functions in species that school and flock, far less is known about the role of UV coloration in group-territorial and colonial species, where individuals have long-lasting and potentially cooperative relationships with many neighbours. We measured spectral reflectance in Neolamprologus pulcher, a group-living, colonial Tanganyikan cichlid fish, and found prominent UV coloration on both sexes. We manipulated the visibility of this UV coloration using a clear UV-blocking filter or a UV-transparent filter during behavioural contests across a barrier. We found that trials between similarly sized same-sex opponents involved significantly lower frequencies of aggression when …