ABSTRACT: Egg production rates (EPR) by Calanus helgolandicus females were investigated with specimens sampled weekly, from April to November 2003 and from March to October 2004, at a station located in the English Channel off Roscoff. Comparison of results between 1994, 2003 and 2004 showed that C. helgolandicus was a late spawner in 1994 and became an early spawner in 2003 and 2004. In all cases high variations in EPR were observed, which could not be correlated to phytoplankton biomass, expressed as diatom, chlorophyll a, particulate carbon and nitrogen concentrations in 2003 and 2004. Neither were they correlated to food quality, expressed as C/N ratio. To explain this mismatch between EPR and food concentration, a series of mixed phytoplankton species dominated by diatoms (≥11 μm filtrate representing natural diatom assemblages: NDA) and 7 single diatom species, all occurring during blooms in the field, were assayed as diets with C. helgolandicus females. Ingestion of diatoms by females was estimated by faecal pellet production rates and complementary scanning electron microscopy examinations of diatom remains in pellets. Depending on diatom species in diets, EPR was either increased or depressed 2 to 3 d after food uptake by females had started. The EPR decrease was reversible, when diatom diets were replaced by the dinoflagellate Prorocentrum minimum. This effect was also observed when females were transferred to natural phytoplankton populations from the English coast of the Channel close to Plymouth, where food composition in the field differed compared to that off Roscoff. EPR ceased completely when the concentration of NDA diets was artificially increased, but recovered after a shift to a dinoflagellate diet. These results indicate that phytoplankton dominated by diatoms can impair C. helgolandicus egg production in the field. This effect was not related to the production of polyunsaturated aldehydes by diatoms. Limitations due to unidentified essential compounds not provided by the metabolism of diatoms, or unknown diatom-derived toxins, were probably involved.