Abstract The seasonal distribution of Prorocentrum lima within the epibiotic community associated with wild and cultured shellfish was studied at eight sites in northeastern USA coastal waters. This dinoflagellate produces toxins that can potentially accumulate in shellfish and lead to diarrhetic shellfish poisoning. Four sites in semi-sheltered environments provided sufficient data for an evaluation of population dynamics. The two southernmost populations (Point Judith Pond and Bluff Hill Cove in Rhode Island) displayed a double peak in abundance, one from March to June, with several thousands cells per g dry weight of collected epibiota, and a second one, sometimes minor, in autumn. At the two northern stations (New Meadows River and Clam Cove in Maine), P. lima populations also peaked twice but with a delay in timing. All four sites harbored filamentous seaweeds and aggregations of tube-forming or chains of diatoms, providing a favorable habitat for this epiphytic dinoflagellate.