A central problem in ecology is determination of the primary biotic and abiotic factors limiting the distribution and abundance of organisms. Estuarine organisms have often been considered to be more subject to control by abiotic than biotic variables (Dunson and Travis, 1991, 1994). The sheepshead minnow, Cyprinodon variegatus, is a primarily herbivorous fish found in salt, brackish, and hard fresh waters. Inland populations are much more common in Florida than elsewhere, occupying several lakes of the upper SaintJohn's River system (Guillory and Johnson, 1986), Lake Okeechobee (Ager, 1971), and the Everglades (Loftus and Kushlan, 1987). Although Martin (1972) concluded that C. variegatus is excluded from fresh water by competition with primary freshwater fish, it is found in the Everglades where numerous freshwater species also occur (Loftus and Kushlan, 1987). Here we test the hypothesis that C. variegatus is excluded from most fresh waters because it is