Populations of the guppy Poecilia reticulata in Trinidad vary markedly in their tendency to school. In many cases this variation in behaviour can be attributed to variation in the predation regime: guppies that co-occur with the pike cichlid, Crenicichla alta, spend more time schooling and form larger schools than their counterparts from low-risk habitats. However, the association between schooling tendency and predation risk is not ubiquitous. In this paper we document the behaviour of guppies from populations in two Trinidad drainages. Guppies from the (Lower) Aripo River (in the Caroni drainage) display well-coordinated schooling behaviour irrespective of whether they are observed in the wild or raised under standard conditions in the laboratory. By comparison, Oropuche River guppies (from the Oropuche drainage) show only a weak schooling tendency. The contrast between the two populations is apparent even in newborn guppies. As pike cichlids are abundant at both sites it seems unlikely that reduced predation risk can account for the weaker schooling of the Oropuche River fish. The behavioural differences in the two drainages are paralleled by considerable genetic divergence and we therefore consider the possibility of phylogenetic constraints on the evolution of schooling.