A system is described consisting of a chemostat for culturing algae, a dilution unit and special experimental vessels, allowing long-term experiments with filter feeders at a constant concentration and quality of food. The algal suspension flows continuously from a chemostat, is mixed with sterile filtered water, and pumped directly into the experimental vessels. The flow through the vessels is intensive enough to avoid a significant change in food concentration by sedimentation or grazing. There are two possible arrangements, the first providing many duplicates with exactly the same food concentrations, the second providing a series of graduated food concentrations. Details of construction are given. The application of both arrangements is demonstrated by experiments with Daphnia pulex. Further possibilities of application are suggested.