Local distribution of Cyprinodon variegatus Lacépède was examined with special emphasis on entrance into fresh water. One hundred and twenty localities were examined in relationship to fish faunas and water chemistry. The chemical factors examined were calcium, magnesium, sodium, potassium, chloride, alkalinity and dissolved oxygen. These factors singly or in combination had little effect on local distribution. Primary freshwater fish and especially centrarchid fishes seem to exclude Cyprinodon with the mechanism for this exclusion appearing to be competition for some resource(s).