The activities of the enzymes a and b-glucosidase, leucine aminopeptidase and alkaline phosphatase were measured in cultures of the dinoflagellate Prorocentrum minimum (Pav.) J. Schiller, and in field samples collected during dinoflagellate blooms in tributaries of the Chesapeake Bay. Activities were measured using fluorogenic artificial substrates and partitioned among different size fractions. Little to no glucosidase activity was found associated with the dinoflagellate size fraction in either cultures or in field samples. A large fraction of the total aminopeptidase (LAP) activity in cultures was found in the dinoflagellate size fraction. LAP activity was positively correlated with dinoflagellate abundance and chl a, and negatively correlated with ammonium concentration. Alkaline phosphatase exhibited a strong response to P-deficiency, increasing up to 330 times in P-depleted cells compared to P-replete conditions, and decreasing after addition of phosphate. P. minimum can make a substantial contribution to ectocellular hydrolytic activity in the water column during blooms.