When fish inspect a predator they incur risk. One way of reducing the cost of inspection is to approach predators in groups large enough to benefit from the safety in numbers advantages of schooling. In a survey of nine populations of guppies, Poecilia reticulata, in their native streams in Trinidad, we observed marked variation in schooling behaviour. Guppies from sites also inhabited by a major predator, the pike cichlid, Crenicichla alta, devoted more time to schooling than those from less dangerous locations where the cyprinodont Rivulus hartii was present. We found a strong correlation between schooling tendency and the group sizes adopted by guppies inspecting a realistic model predator. Since guppies in dangerous localities approached a potential predator in large groups it seems unlikely that many of these fish were caught in a Prisoner's Dilemma. Inspections by singleton fish were rare in high-risk locations but predominated in those populations where risk from fish predators was reduced.