We have described the occurrence of 46 phytoplankton species that are potentially toxic to humans, or harmful to marine life, or both. The area of interest is southeastern Nova Scotia to the Hudson River (NY) estuary. The species are distributed across a number of taxonomic classes, and represent a conservative estimate of the real total, which must remain speculative until the biodiversity of phytoplankton in the area is known. Despite the high number of potential harmful algal bloom (HAB) species, rather few are known to cause problems.