The reproductive biology of Calanus finmarchicus was investigated in the Norwegian Sea along a transect from 62°22'N, 5°12'E to N 64°N, 0°0'E, which was sampled twice in April 1997 with RV G.O. Sars. The aim of this study was to compare the timing and the magnitude of spawning on the shelf and in the open sea. Egg production rate, clutch size, gonad development stage and sex ratio of C. finmarchicus were determined at 29 stations from WP 2 net samples in the upper 100 m. Gonad maturity and egg production varied according to hydrography: High proportions of mature females and maximum egg production rates of up to 96 eggs female d were observed on the shelf where an early spring phytoplankton bloom provided optimum food conditions. In the Atlantic water of the open sea, the proportion of mature females and the egg production rate were generally lower but reached local maxima.