The diversity of zoo- and phytoplankton in alpine lakes has been studied intensively for over 100 years. It is now generally accepted that the species diversity of plankton communities decreases with increasing altitude. This study addresses the question as to whether or not this general pattern also applies at the population level. Daphnia , a typical member of zooplankton communities, was chosen as the study organism. Genetic diversity was determined for 11 Daphnia populations in mountain lakes at varying elevations. We found that the genetic diversity was very heterogeneous.