In the 1990s, a period of extraordinarily mild winter temperatures in the German Bight (North Sea), the oceanic, neustonic isopod Idotea metallica Bosc was found for the first time off the island of Helgoland. The species was recorded in subsequent summer periods, reproducing successfully in the area, while its whereabouts in winter remained uncertain. The question arises whether a continuing increase in mean winter water temperature, as predicted in most scenarios on future climate, might enable I. metallica to overwinter in the North Sea and to become a permanent resident there. Experiments on laboratory-cultured I. metallica were performed for the first time. Population dynamics was studied at different temperatures in microcosms. Furthermore, the temperature effects on reproductive output, mortality and duration of embryonic development were studied in individually reared animals. The results suggest that at current temperature conditions (mean winter water temperature of about 5°C) I. metallica is unable to overwinter in the German Bight, and that even an increase to about 8°C probably would not change this situation. The recently observed summer populations of the species in the German Bight obviously originate from individuals introduced each year anew by water currents from the Atlantic Ocean. Nevertheless, I. metallica may be useful as a sensitive indicator: in a warming North Sea the species may become a regular summer immigrant, developing more and more conspicuous populations in this area as the annual period with water temperature above the critical level (13°C) for reproduction extends. © 2001 Elsevier Science B.V.