A permanent pond at 3100 meters elevation was studied for an entire year to monitor zooplankton populations as they changed seasonally and associate those changes with biotic and abiotic factors. Conductivity, alkalinity, and dissolved solids were low during the summer but increased dramatically in the winter due to “freezing out.” Chlorophyll measurements indicated that phytoplankton reached its population maximum during the winter. The summer zooplankton community was dominated by the cladocerans,Holopedium gibberumandDaphnia rosea; the copepod,Diaptomus leptopus; the rotifers,Keratella cochlearis, Conochilus unicornis, andPolyarthra dolichoptera; andChaoboruslarvae.D. roseadisappeared from the plankton in August, possibly due to a combination of predation and competition. All limnetic Crustacea disappeared during ice cover, leaving rotifers as the most successful winter plankters. The posterior spine ofK. cochleariswas long in the summer, and short or absent in the winter. This seasonal change in morphology may be a means of reducing mortality due to predation.