ABSTRACT: The accessory pigment alloxanthin is a well-known taxonomic marker for Cryptophyceae in natural seawater. The use of alloxanthin to estimate in situ zooplankton grazing and selectivity on Cryptophyceae was studied using 2 dominantcopepods of the southern North Sea, Temora longicornis and Centropages hamatus. High-performance liquid chromatography (HPLC) analyses of water samples and of freshly caught copepods, starvation experiments, and feeding experiments(shipboard bottle-incubations) were carried out. Although alloxanthin was always detected in copepod extracts, (1) no correlation was found with alloxanthin in seawater, and (2) no significant grazing on Cryptophyceae was observed. Results of ourgut-evacuation experiments showed that at least 78% of the initial alloxanthin content of T. longicornis and C. hamatus remained after 90 min, whereas fucoxanthin and chloropigments decreased rapidly with time. Alloxanthin and astaxanthinesters were the only pigments remaining in the body of T. longicornis after several hours¹ starvation. It is concluded that most of the detected alloxanthin did not originate from the gut but from the body tissues. Our results suggest thatalloxanthin is not suitable as a biomarker for quantitative or qualitative estimates of copepod grazing on Cryptophyceae.