Summary M. Mathematical descriptions of uniform and non- uniform phytoplankton grazing by zooplankton are presented and equations for calculating the dependence of phytoplankton production on rate and rhythm of elimination are deduced. 2. of greatest interest is non-uniform phyto- plankton grazing, which is reflected in a sinusoidal eurve and is usually found in a bath eco- system with migratory consumers 3. Phytoplankton production depends upon the time of grazing; the nearer towards nightfall grazing oceurs, the higher is the phytoplankton production. 4. In order to be able to calculate the phyto- plankton production and the amount of food consumed under conditions of non-uniform sinusoidal grazing,