A combination of shipboard and laboratory experimentation and mesocosm rearing is used in an integrated study of feeding, growth, and reproduction of Calanus finmarchicus. Comparisons are made where possible with the co-occurring Calanus helgolandicus. Regional and seasonal variability of feeding and reproduction was investigated for C. finmarchicus in the open Norwegian Sea and for C. helgolandicus at a coastal station in the English Channel. Mesocosm experiments, carried out in western and northern Norway to identify environmental cues governing the life-cycle pattern, are compared with the results of laboratory studies of nutrition, growth, and reproduction under controlled conditions. The results are discussed in relation to physiological and life-cycle models of Calanus. © 2000 International Council for the Exploration of the Sea.