ABSTRACT: Pelagostrombidium fallax from surface waters of Lake Constance, Germany, was cultivated at 9°C with Rhodomonas sp. as food. Flask-shaped resting cysts typical for oligotrich ciliates were frequently observed in the stock cultures. In batch culture experiments, encystment occurred at high and low food levels. Cyst formation ceased during phases of population growth, whereas enhanced encystment was recorded shortly after food had been reduced to low concentrations by ciliate grazing. In any situation, only a fraction of the population encysted.