We conducted three surveys of the coastal and offshore waters of the northern Gulf of Maine between New Hampshire and the outer Bay of Fundy during the summer of 1998, collecting data from more than 200 stations during each cruise in June, July and August. Hydrographic data were collected and concentrations of phytoplankton chlorophyll, inorganic nutrients and cell densities of Alexandrium, the dinoflagellate responsible for Paralytic Shellfish Poisoning (PSP), were measured in discrete water samples. The distributions of Alexandrium on all three cruises displayed maximum cell densities in the offshore surface waters of the Gulf, and not immediately adjacent to the shoreline where PSP toxicity in shellfish is typically reported. Highest cell densities (ca. 5,500 per liter) were observed in two broad patches: one in the Bay of Fundy and another in waters offshore of the central and eastern Maine coast. Examined against satellite images of sea surface temperature, highest cell densities were well correlated spatially with waters having experienced vertical mixing by tides (i.e., the Eastern Maine Coastal Current/Plume system, and western Bay of Fundy waters). As the summer progressed, the highest densities of Alexandrium appeared to recede toward the eastern portions of the Gulf and the Bay of Fundy. We suggest that the naturally-occurring offshore distributions of relatively high densities of Alexandrium in surface waters can be related to dynamics controlling inorganic nutrient fluxes and the ambient light field as it varies seasonally and vertically. We also suggest that periodic outbreaks of PSP in nearshore waters are caused by wind-driven advection of cells to those shellfish beds.