Copepods are able to discriminate between different foods on the basis of particle size and nutritional quality. However, the extent of selective feeding behavior and the mechanisms controlling it in the field are still poorly understood. In this study, we investigated selective feeding behavior and egg production for Calanus helgolandicus feeding on natural phytoplankton (using high-performance liquid chromatography techniques), and egg production, at a coastal station off Plymouth with the annual phytoplankton cycle from July 1996 to June 1997. The phytoplankton succession included biomass peaks of dinoflagellates, prymnesiophytes, and diatoms. C. helgolandicus showed little selective feeding behavior throughout the study with a slight preference for diatoms. The influence of the diet composition on egg production was analyzed using forward stepwise regression methods. Prymnesiophytes and diatoms were shown to have positive effects whereas the effect of dinoflagellates was negative. The effect of the different phytoplankton peaks is analyzed and discussed in relation to the phytoplankton taxonomic composition and dietary diversity.