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Devils Hole lip level, and 2) roof collapse after the last excursion of water table above Devils Hole lip level. In the first scenario, organisms migrating up a surface stream from Ash Meadows would colonize Devils Hole pool, and isolation would start with the last fall of water table below the lip. According to Winograd and Szabo (these Proceedings) that probably would have occurred between 180 and 800 Ka ago. In the second scenario, Devils Hole pool forms after water levels became low enough that no surface stream would ever connect Devils Hole with Ash Meadows, and above ground colonization would take place over dry intervening countryside. In this case, the time of isolation would begin with introduction of the pupfish, sometime after roof collapse. At the present time there is insufficient evidence to choose between these alternatives, however several observat-ions may be significant: 1) on the basis of two samples only, travertine precipitation on the walls of the main chamber stopped about 50 Ka ago, after operating essentially uninterruptedly for the prior 700 Ka, and 2) ceiling collapse floored the main chamber with bedrock breakdown that is uncoated by travertine, and was therefore emplaced 50 Ka or less ago, and 3) scanning electron micrographs show a layer of what appears to be organic material coating travertine surfaces in the main chamber. This coating may be what stops travertine precipitation even though the water remains supersaturated. Together, these observations suggest that surface collapse took place about 50 Ka ago, fostering the development of a photosynthetically based, surface bound community that prevented further travertine precipitation and produced an environment favorable for pupfish colonization. 48