The guppy, Poecilia reticulata, exhibits marked intraspecific variation in behaviour and morphology and occurs over a wide range of natural environments in Trinidad, West In-dies. In this paper we examine the hypotheses that: 1) in clear mountain headstream waters in Trinidad, where Rivulus hartii is the only major aquatic predator, sexual selection has favoured the evolution of visual components of courtship behaviour in the guppy, and 2) in turbid lowland rivers with numerous aquatic predators selection has resulted in greater dependence upon non-visual components of courtship. Guppies were observed in four rivers in Trinidad. In two mountain headstreams guppies were evenly distributed across and along the river. Guppies in a lowland river and a 'midstream' river occurred in small schools close to the water's edge. Males guppies in the headstreams performed more frequent sigmoid displays and displays of longer duration than males in the turbid lowland river and the midstream river. Males in the turbid lowland river were found to exhibit higher frequencies of gonopodial thrusts than males in the other three rivers. Offspring of guppies from the four rivers were raised under identical conditions in the laboratory, free from predation and other selective agents. The results of observations of courtship behaviour in laboratory-raised fish are consistent with those obtained in the field. The overall similarity of results in the field and the laboratory provides evidence that there is a genetic basis to the observed behavioral differences between populations.