Censuses of six tern species were carried out in the inlet of Lagoa dos Patos, southern Brazil, and on the beaches just north (along 50 km) and south (60 km) from this site. The Common Tern (Sterna hirundo) was the commonest species between October and March, with maximum abundance in January (14,100 individuals). Abundance peak for the South American Tern (Sterna hirundinacea) took place in the austral winter, with maximum values of 1,500 birds. Trudeau’s Tern (Sterna trudeaui) and Amazon Tern (Sterna superciliaris) had similar seasonal patterns occurring during all months, with abundance peak in the non-breeding period, between January and July. The maximum number of Trudeau’s Tern was estimated to be 440 birds in April and 400 in June, and that of Amazon Tern was 210 birds in April and 230 in May. A maximum of 300 Royal Terns (Sterna maxima) were recorded in September, 280 in June and 280 in July, but the species was found in every month. The Cayenne Tern (Sterna eurygnatha) was also found every month, and the maximum number occurred in October (61 terns). All six species used the southern Brazilian coast, predominantly during the non-breeding season. Common, South American, Cayenne, and Royal Terns find their breed far north and south of the study area. Trudeau’s Tern and the Amazon Tern breed inland in southern Brazil. Only the Amazon and Royal Terns showed preference for particular beach sub-areas, and both were found in large flocks near the Lagoa dos Patos inlet, the former nearer to freshwater, and the latter associated with discharges from the artisanal fishery. Beaches are used by Common Terns as roosting sites during daylight hours, and the Lagoa dos Patos inlet is used mostly for night roosting.