Carbohydrate macromolecules (dextrans) labeled with fluorescein isothiocyanate (FITC) were taken up by the dinoflagellate Alexandrium catenella at a substrate concentration of 5 mg C l(-1) The FITC-labeled dextrans appeared in what resembled food vacuoles inside the dinoflagellate cells. Between 5 and 50% of the cells contained fluorescent green vacuoles. A. catenella took up dextrans of high molecular weight (2000 kDa) but did not show significant uptake of lower molecular weight (20 kDa) dextrans. The uptake of the 2000 kDa dextran was higher with addition of humic substances to the growth medium and did not seem related to the presence of bacteria. Phagocytosis of fluorescent microspheres (0.36 mu m) by A. catenella was also investigated. Although aggregation of fluorescent microspheres was observed in the sulcal region of the cells, no evidence was found of phagocytosis of bacterial-size prey by A. catenella. These observations show that A. catenella has the capacity to take up high molecular weight organic molecules. perhaps by pinocytosis.