Spatial and seasonal egg production rates (E-r) and egg hatching success in the copepods Calanus finmarchicus and Calanus helgolandicus were measured in the North Sea from March to September. Food availability was monitored by chlorophyll and protist concentrations and three size fractions of seston fatty acids. Seasonal and spatial distribution and production differed between the species. Calanus finmarchicus was found only offshore of the 50-m isobath, with decreasing E-r (37-28 eggs female(-1) day(-1)) from March to July. Calanus helgolandicus had two abundance peaks, in spring and autumn, with a low in May during which time the highest E-r were observed (38 eggs female(-1) day(-1)). At other times, E-r in C. helgolandicus remained lower than in C. finmarchicus (similar to 20 eggs female(-1) day(-1)). Normalized E-r (E-n) in both C. finmarchicus and C. helgolandicus were positively related to ciliate biomass while only negative relationships were found for all other variables measured. Hatching success in both Calanus species combined was significantly correlated with the essential fatty acid ratio 22:6n3/20:5n3.