In many ecological systems, community structure changes radically with the seasons. Seasonal changes are important because of their effects on both biotic and abiotic factors. In an attempt to address the question of whether ostracods exhibit variation in their seasonal occurrence (i.e., seasonally), twenty freshwater ostracod species collected monthly from seven springs in Nevada-U.S.A. was studied over a year. The seasonal distribution of some species in different genera (Prionocypris longiforma and Cavernocypris wardi, Darwinula stevensoni and Ilyocypris bradyi) showed similar occurrence all year around in rheocrene springs. Finding some species (e.g., Darwinula stevensoni and Ilyocypris bradyi) from the same locality suggests differences in their food requirement and life histories. This is maybe because species of different genera can avoid competition by preferring different types of food and habitats in different seasons. These results suggest that the diversity, abundance, and occurrence of the spring ostracods are seasonal, and sampling from an area at one time only may not illutstrate the complete fauna! richness of that area.