The calanoid copepod Temora longicornis occurs year-round in the southern North Sea. Owing to its limited ability to store energy, this species relies on a constant food supply. Thus, when the phytoplankton stock as the primary food source is low in winter and early spring, T. longicornis needs to utilize other food items. In this study, the feeding strategy of T. longicornis females is elucidated using a broad methodological approach. From March to May 2005 we conducted weekly grazing experiments using natural plankton (<70 μm) as food source. In addition, gut contents, stable isotopes (∂15N, ∂13C) and fatty acid composition of the females were analysed, as was the natural plankton composition. Our data suggest that females were omnivorous during late winter and early spring, switching to a more herbivorous feeding mode with increasing phytoplankton stock in spring. The ∂15N values in March/April are the highest reported so far for this species, and we suggest that they mirror the copepods' trophic level throughout ontogenesis rather than reflecting the feeding history of only the females. T. longicornis adjusted its feeding mode to ambient food conditions, utilizing a broad range of food sources. Experiments, however, revealed strong selection for cells >12.5 μm. Feeding on dinoflagellates was generally intense; diatoms were ingested mainly in March. Thus, the grazing impact of T. longicornis can be substantial on those taxa, which are selected for. © The Author 2008. Published by Oxford University Press. All rights reserved.