<jats:p>Coastal waters of the northeastern U.S. are subject to recurrent outbreaks of paralytic shellfish poisoning (PSP) caused by toxic dinoflagellates in the genus <jats:italic>Alexandrium.</jats:italic> PSP is not uniform across the large region, but instead reflects <jats:italic>Alexandrium</jats:italic> growth and toxin accumulation in five separate habitats or zones defined by circulation patterns and the discontinuous distribution of the dinoflagellates. Each of these habitats has a unique set of environmental and oceanographic forcings that determine the timing and extent of bloom development and transport and that regulate the extent of genetic exchange with adjacent populations. Several habitats (e.g. the southwestern Gulf of Maine, Massachusetts Bay, and Georges Bank) are linked hydrographically and may share the same <jats:italic>Alexandrium</jats:italic> population via large‐scale transport in a. coastal current, whereas the other two habitats (eastern Maine and southern salt ponds‐embayments) seem to be isolated and have little or no hydrographic or genetic linkage to adjacent regions during bloom seasons. My paper provides an overview of the regional ecology and oceanography of <jats:italic>Alexandrium</jats:italic> through a focus on these five subpopulations. Issues that relate to PSP and <jats:italic>Alexandrium</jats:italic> dynamics throughout the world are highlighted, including species dispersal, the role of cysts and “initiation zones” in bloom development, and the influence of large‐ and small‐scale hydrography on population development and transport. The ability of <jats:italic>Alexandrium</jats:italic> to colonize multiple habitats and to persist over a large region is emphasized in recognition of the adaptability and resilience of this important organism.</jats:p>