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26 sefiala una poblacion de casi 2750 individuos en la cienega. Ninguna de las dos ocurre en numeros significantes en otras partes de la charca (basado en 50 cuadrados). Desde la introduction de los robalos, los cyprinodontidos se han considerado amenazados, particularmente en el caso de Megupsilon. En mayo de 1978, la situation habia mejorado, pero esto po idid a.haber sido una respuesta temporaria al tiempo que estaba excepcionalmente pluvioso en aquel afio. E1 esfuerzo para eliminar los robalos de El Potosi con redes no ha tenido exito. Parece imprudente emplear metodos mas efectivos porque los cyprinodontidos y otros organismos endemicos pueden ser afectados adversamente. Por eso, la conservation debe ser basada en el manejo del medio ambiente. Las investigaciones sobre la separation. ecologica y las requirimientos ambientales de los cyprinodontidos estan en desarrollo como base de un planeamiento del manejo de los ambientes. THE STATUS OF MEGUPSILON APORUS AND CYPRINODON ALVAREZI AT EL POTOSI, NUEVO LEON Two unique endemic fishes inhabit a spring-fed pond at E1 Potosi, Nuevo Leon, in northeastern Mexico. The pond is part of an isolated system of springs which rise on a high arid plateau about 125 km south of Monterrey. This limited habitat is probably the last remnant of a larger aquatic system which may have covered much of the plateau during Pleistocene pluvial periods (Miller and Walters 1972). Just as the pond is a vestige, so are its native species. They are the only surviving relicts of an earlier, more complex aquatic biota. Although the native fishes of E1 Potosi have survived Pleistocene climatic changes and a severe reduction of their habitat, they are now facing extinction by predatory fish which were introduced into the pond in the early 1970's. Their plight is further complicated by structural modifications of their habitat. Four species of fishes now occur at E1 Potosi. Cyprinodon alvarezi is an endemic pupfish (Cyprinodontidae) which is closely related to the C. eximius complex (Miller 1976). Megupsilon aporus constitutes a monotypic pupfish genus which is also endemic to El Potosi (Miller and Walters 1972). Goldfish, Carassius auratus, have been present in the pond since sometime prior to 1961, and largemouth bass, Micropterus salmo-ides, were introduced about 1974 (Contreras-Balderas 1978). The aquatic habitat at E1 Potosi has been highly modified to provide water for domestic and agricultural use. A pond has been created by construction of walls and dikes around one of the spring areas. Four springs rise at the base of one of the dikes and issue into a system of irrigation ditches which extends several kilometers into adjacent fields. An additional spring lies north of the pond and also connects with the irrigation system. No habitat remains in its