This paper is concerned with mathematical considerations of the dependence of phytoplankton production rates upon rhythm and rate of elimination by zooplankton. A mathematical model is tested, using field data obtained on a Black Sea bathyplankton comunity as an example. Furthermore, the influence of phytoplankton uptake of a Calanus helgolandicus lULAUS) (the main bathyplankton species) population on phytoplankton production was investigated. Various zooplankton representatives were studied under both laboratory and field conditions, and certain feeding rhythms observed. organisms which do not undertake considerable migrations and practically inhabit only the upper layer, have a relatively high rate fo feeding during the whole day; their feeding rate is fairly steady throughout 24 h, with two maxima at midday and midnight. This type of feeding is characteristic of most epiplankton species and of non-migratory bathyplankton juveniles Marine Biology, vol. 3