Spatial and temporal patterns of Calanusfinmarchicus and C. helgolandicus (Copepoda, Calanoida) were investigated in the northeast Atlantic and the North Sea from 1962 to 1992. The seasonal cycle of C.finmarchicus is characterised by a single peak of abundance in spring, whereas the seasonal cycle of C. helgolandicus shows 2 abundance maxima, one in spring and one in autumn. The former species mainly occurs in northern regions (limited by the 55*N parallel in the North Sea and by the 50*N parallel in the open ocean). The latter species shows 2 types of spatial patterns, occurring in the Celtic Sea during spring and in the Celtic Sea plus the North Sea in autumn. Differences in seasonal spatial patterns of Calanus species may result from different responses to the environment, ultimately due to different life cycle strategies, different vertical distributions, opposite temperature affinities and interspecific competition. Futhermore, results reveal that annual means of abundance are closely related to annual maxima and to spatial extensions of the species. It also appears that the long-term trends of the 2 Calanus species are opposite: C. finmarchicus shows a clear downward trend in abundance, while C. helgolandicus presents an upward one.