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27 natural state. Although the original habitat is unknown, it seems likely that it was a marsh since the springs rise in a generally flat area. The pond is shallow, about 3 m at its deepest point, and its total surface area varies greatly as water levels fluctuate. At its greatest extent, the pond is about 150 m long and has a surface area of approximately 4500 m2. This includes 1450 m2 of marsh with an average depth of 10-20 cm. This shallow area provides heavy cover for pupfish in the form of patches of narrow-leaved and broad-leaved Potamogeton, Nasturtium, and sedges. In May, 1978, the marsh supported the densest pupfish populations, and breeding territories of both Me u silon and Cyprinodon were observed there. An area of about 2100 m is covered with a dense mat of Ceratophyllum. The remainder of the pond, 950 m2, is open water. When R.R. Miller visited El Potosi in 1961, pupfish were abundant in open water around the central pool. Goldfish were also present and may have been there for some time; there is no indication that they have a negative impact on the pupfish. However, bass have become abundant since their introduction, particularly in deeper areas and the Ceratophyllum mat. The pupfishes have disappeared from those areas and are now restricted to the marsh and irrigation ditches where bass are absent. Water leaves the pond through a concrete sluice about 0.5 m deep, which is deep enough to completely drain the marsh. The pupfish are therefore in a difficult situation. When water levels drop each summer, the pupfish are caught between the drying shallows and preda- tion in deeper water. In May, 1977, the marsh was completely dry and only a few pupfish could be found along the margins of the main pool. In May of the next year, the marsh was flooded again, and pupfish were abundant. The depletion of stocks in the pond might not have an immediate impact on the status of C. alvarezi since it is abundant in the springs and ditches outside the pond. However, Megupsilon may not be capable of long-term survival in those areas, It was not known to occur in the springs until it was introduced there by Salvador Contreras-Balderas in 1976. So far it has held on in one spring and some of the ditches. Earlier it had appeared that Megupsilon was excluded from those habitats by competition with Cyprinodon. We do not yet know much about ecological segregation between the two pupfish species. C. alvarezi has a long gut, about twice its standard length, and filamentous algae are the dominant component of its diet. M. aporus has a shorter gut, about 80% its standard length, and feeds primarily on invertebrates (Miller and Walters 1972). It has been seen feeding at the surface.