In both the New and Old World, endemism in freshwater fishes of the family Cichlidae presents novel problems in differentiation and speciation (Fryer and Iles, 1972; Trewavas et al., 1972; Greenwood, 1974). In the New World, at least three undescribed forms are endemic to the Cuatro Cienegas Basin, Coahuila, Mexico (Taylor and Minckley, 1966; Minckley, 1969; La Bounty, 1974). The most abundant and widely distributed of the endemic forms possesses very fine pharyngeal teeth and light supporting musculature, and feeds primarily upon algae and detritus. Occurring with less abundance and more limited distribution, is a form characterized by thick heavy pharyngeal teeth used primarily for feeding upon snails. Upon external examination, the two forms are indistinguishable. The distribution and abundance of a third endemic is very restricted. It is presumably predatory in habit, and differs markedly in shape and dentition from the other two types (Taylor and Minckley, 1966; La Bounty, 1974). The occurrence of these endemic fishes within a limited geographic area is reminiscent of certain African lake cichlids (Fryer and Iles, 1972). Although the ancestoral form is not known, a widely distributed species, Cichlasoma cyanoguttatum (Baird and Girard) has invaded portions of the Cuatro Cienegas Basin in recent years, and occurs sympatrically with the endemic species in several places. Hybrids between C. cyanoguttatum and the two endemics are rare (La Bounty, 1974).