The relative abundance of the most common cetacean schools in the eastern tropical Pacific Ocean for 1977–1980 are estimated based on encounter rates with tuna purse-seiners. No temporal trends were apparent in the relative abundance estimates. The geographic distributions for eight different school types are described. Multivariate statistical techniques are used to investigate interrelations between species and relationships to parameters of the physical environment. The results suggest three major species groupings: (1) an inshore grouping of bottlenose dolphins (Tursiops truncates), Risso's dolphin (Grampus griseus), pilot whales (Globicephala macrorhynchus) and, to a lesser extent, common dolphins (Delphinus delphis); (2) an offshore pelagic grouping of spotted and spinner dolphins (Stenella attenuate and S. longirostris); and (3) an association between pilot whales and common dolphins that overlaps the first grouping in inshore areas and also tends to be segregated from the second grouping. The results also suggest that relative densities of different school types are strongly related to physical environmental parameters, the most important being sea surface temperature, depth of the thermocline and thickness of the oxygen minimum layer.