Diarrhetic shellfish poisoning (DSP), a gastrointestinal disorder, is caused by the ingestion of shellfish contaminated with okadaic acid (OA) or one of the dinophysis toxins (DTX-1-3). The digestive gland of blue mussels (Mytilus edulus) collected from sites along Eastern Bay and Frenchman Bay, Maine displayed protein phosphatase inhibition activity. Subsequent analysis of these mussels by LC-MS/MS showed low amounts of DTX-1. To determine the source of this toxin, both phytoplankton samples and epiphytic samples were collected. Analysis of phytoplankton rich in Dinophysis norvegica, a known toxic species, showed no protein phosphatase inhibition activity, OA or DTX-1. However, analysis of macroalgal samples rich in the benthic dinoflagellate Prorocentrum lima displayed protein phosphatase inhibition activity and the production of DTX-1. Cultures of P. lima initiated from these macroalgal samples produced DTX-1 and small amounts of OA. This is the first study to link the toxic benthic dinoflagellate Prorocentrum lima with toxic shellfish in the Eastern United States.