In early June 1988, an extensive bloom of Phaeocystis sp. (29xlO6 cells 1−1) was observed in the eastern Irish Sea along the North Wales coast. Chlorophylls, carotenoids, free amino acids, fatty acids, vitamin C, carbon and nitrogen were measured in particles from three size categories (<22 μm, 22–100 μm and 100–200 μm) at a station 13 miles off the coast line. Phaeocystis sp. formed 99% of the total particle stock below 22 μm and was characterized by (1) the presence of chlorophyll c3, (2) the dominance of fucoxanthin over 19'- acyloxyfucoxanthins, (3) the presence of C18:5 fatty acid at trace levels and (4) different proportions of amino acids compared to those measured in particles greater than 22 μm. These larger particles were dominated by diatoms, dinoflagellates and ciliates. The poor nutritional value of Phaeocystis sp. for herbivorous zooplankton was characterised by its low content of polyunsaturated fatty acids and vitamin C. Comparison between the chemical composition of Phaeocystis sp. and copepod faecal pellets showed that the copepods were feeding predominantly on other phytoplankton.