Notes: Abstract Twenty-eight strains of toxic dinoflagellates in the genusAlexandrium from the northeastern United States and Canada were characterized on the basis of morphology, bioluminescence capacity, mating compatibility, and toxin composition. The distributions of these characters were evaluated in the context of regional patterns of paralytic shellfish poisoning (PSP) and coastal hydrography. Two morphospecies were identified-A. tamarense Lebour andA. fundyense Balech. The two are interspersed geographically though there are areas, such as the Gulf of Maine, where apparently onlyA. fundyense occurs. Southern waters (Cape Cod, Connecticut, and Long Island) have especially diverse populations. The two species are sexually compatible. Virtually all northern isolates are bioluminescent, whereas southern isolates include bioluminescent and non-bioluminescent strains. Cluster analyses, based on high performance liquid chromatography (HPLC) determinations of the suite of toxins produced by each isolate, revealed two and perhaps three distinct groups. One is comprised almost exclusively of northern strains, and the other of southern strains. A Cape Cod cluster may be separable from the southern group. These analyses explain a previously reported north-to-south trend of decreasing toxicity, as the northern isolates produce greater proportions of the more potent toxins than do southern forms. The overall perspective is that the biogeography of toxicAlexandrium spp. in the study region is not that of a single, widespread, homogeneous population, but rather is comprised of several sub-populations, each with its own physiological characteristics and history. Two scenarios are considered with respect to this regional biogeography. The first invokes recent and continuing dispersal of isolates to the south from a center of origin in the north, followed by recombination and strong selection. The second holds that the northern and southern populations diverged from a common ancestor (vicariance), but now represent localized populations with little mixing of genotypes. Neither hypothesis can be completely refuted by the data presented here, though the weight of the evidence favors the latter. The correct scenario may be a combination of both, with recent and continuous speading occuring within the Gulf of Maine and perphaps the Gulf of St. Laerence, but with endemic localized populations persisting without genetic exchange in most southern locations. These data also indicate that although morphological criteria separate toxicAlexandrim isolates from the study region into two morphospecies, these assignments do not coincide with clusterings based on toxin composition or allozyme electrophoresis, and they are further violated by mating results. A revision of taxon designations to the varietal level could be justified.