A survey of Notonectidae from aquatic habitats in Guanacaste Province, Costa Rica, is given; 7 species of backswimmers representing 3 genera are reported. A number of environmental parameters are quantified for each site sampled and are used to characterize the habitat of each species. Species of Buenoa are found in a variety of warm pond habitats while Martarega are found in river sites characterized by cooler water and conditions favoring a floating prey resource. Laboratory tests show that M. hondurensis Bare has a lower maximum temperature tolerance than does B. antigone (Kirkaldy); M. hondurensis is also found to have a marked preference for food floating on the surface, while B. antigone prefers those food resources spending the greatest amount of time in the water column. Size ranges of 5 species of Buenoa are found not to overlap significantly. This may be important in minimizing interspecific competition.