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28 The two species seem to be reproductively active at the same time. In February, March, and April (the only months for which specimens are available), the females of both species carry size- graded series of eggs; they are apparently capable of repeated repro- duction over a long period of time. Both species establish breeding territories in the fine vegetation of the marsh, sometimes within 20 cm of each other, and competition for territories may be intense. C. alvarezi is commonly twice the size of M. aporus and is more aggressive. It has been seen to threaten Megupsilon males which were defending territories. The proportion of Cyprinodon to Megupsilon fluctuates greatly, possibly in response to habitat changes. When the pond was visited in 1961, Megupsilon was apparently rare. In 1968, however, there were three times as many Megupsilon as Cyprinodon. In three different years since then, Cyprinodon has been dominant. Population sizes of the two species were estimated in May, 1978, by counting individuals visible in meter-square quadrats in each habitat type. The average density of Cyprinodon in the marsh, based on 73 quadrats, was 8.4 fish per m2 which indicates about 12,180 Cyprinodon in the whole marsh. The average density of Megupsilon was 1.9 fish per m2 which indicates a population of about 2750 individuals in the marsh. These are probably underestimates since many fish were concealed. The proportion of four Cyprinodon to each Megupsilon may be biased toward Cyprinodon, since Megupsilon sometimes takes cover when the former species is present. Neither species occurred in significant numbers in other parts of the pond (based on 50 quadrats). Since the introduction of bass at E1 Potosi, the two pupfishes have been considered threatened, Megupsilon critically so. In May, 1978, the situation had improved, but this may have been a temporary response to an unusually wet spring. Efforts to eliminate bass at E1 Potosi with gill nets have proven unsuccessful. It seems imprudent to use more effective means of bass eradication as the pupfish and other endemic organisms might be adversely affected. Preservation efforts will therefore be based on habitat management with the objective of altering predator-prey interactions in favor of pupfish. Research on life histories, habitat requirements, and ecological segregation is currently under way and will provide the basis for a future management program. Acknowledgements.--Field work was supported in part by grants from the National Science Foundation (DEB77-17315) and the Fauna Preservation Society, London (both to R.R. Miller) and by a Scott Turner Award in Earth Science. Ellie Baker Koon assisted in field work in 1977, and R.R. Miller, Edie Marsh, and Salvador Contreras-Balderas and his students from the Universidad Autonoma de Monterrey participated in 1978. Permission to conduct field work in Mexico was kindly granted by the Direccion General de Regiones Pesqueras.