The ingestion of phytoplankton by Hyas araneus larvae was shown by light- and scanning electron microscopic examination of faeces and HPLC analysis of larvae collected from the field (zoea I) or cultivated in the laboratory (zoea I, II). To date these larvae have been considered to be carnivorous. Three phytoplankton pigments were detected by HPLC: chlorophyll a (marker pigment for phytoplankton in general), fucoxanthin (marker pigment for Bacillariophyceae) and peridinin (marker pigment for Dinophyceae); also present were the degradation products of chlorophyll a (phaeophytin a and phaeophorbide a). The pigment composition of larvae collected from field samples showed that dinoflagellates were ingested in addition to diatoms. The pigment concentration in larvae from field samples was lower than in laboratory-reared larvae, which may be due to lower algal concentrations in the field. The chlorophyll a concentration in the larvae from the field ranged from 0.05 to 0.5 ng·larvae-1, phaeophytin a from 2.1 to 8.2 ng·larvae-1 and phaeophorbide a from 0.1 to 0.4 ng larvae-1. The fucoxanthin and peridinin concentrations varied from 0.2 to 3.8 ng·larvae-1 and from 0.0 to 0.3 ng larvae-1, respectively. Similar values of dry weight and elemental composition in the zoea I were found in field samples and laboratory cultures fed with diatoms exclusively. © 1993.