The winter distribution, seasonal vertical migration, and reproduction of Calanus finmarchicus in relation to water masses and chlorophyll a were investigated southwest of Iceland between November 1996 and June 1997 as part of the TASC programme. During winter (November/December, January/February), overwintering C. finmarchicus were generally located beyond the shelf (5000–30 000 individuals m−2); by contrast, overwintering animals were usually scarce on the banks (2000 m) at temperatures of ∼3–8°C, whereas in the Irminger Basin the distribution was mainly ∼200–1800 m in waters of ∼3–6°C. During winter, most of the stock was present as C5 (84–86%), with a smaller proportion as C4 (12–14%) and females (1–2%). Overwintering animals resided mainly in water of Atlantic origin, but some animals were found in Labrador Sea Water and Iceland–Scotland Ridge Overflow Water. Ascent to upper layers probably took place mainly during March and April, and continued until May. On the basis of the horizontal and vertical distribution of overwintering animals and the general surface current pattern, the banks seem to be repopulated mainly from the south. By April the spring growth of the phytoplankton was about to begin on the banks, and C. finmarchicus had started to lay eggs (0–7 eggs female−1d−1). Maximum individual rates of egg production (2–46 eggs female−1d−1) were observed in June, at the time of maximum phytoplankton biomass accumulation. During April, the population's average rate of egg production on the banks (∼1000 eggs m−2d−1) was rather similar to that in the open ocean (∼600 eggs m−2d−1), whereas during June it was significantly higher in the oceanic region (∼16 000 eggs m−2d−1) than in shelf waters (∼900 eggs m−2d−1), mainly because of the greater number of spawning females in the oceanic environment.