Alexandrium catenella (Whedon et Kof.) Balech, A. tamarense (M. Lebour) Balech, and A. fundyense Balech comprise the A. tamarense complex, dinoflagellates responsible for paralytic shellfish poisoning worldwide. The relationships among these morphologically defined species are poorly understood, as are the reasons for increases in range and bloom occurrence observed over several decades. This study combines existing data with new ribosomal DNA sequences from strains originating from the six temperate continents to reconstruct the biogeography of the complex and explore the origins of new populations. The morphospecies are examined under the criteria of phylogenetic, biological, and morphological species concepts and do not to satisfy the requirements of any definition. It is recommended that use of the morphospecies appellations within this complex be discontinued as they imply erroneous relationships among morphological variants. Instead, five groups (probably cryptic species) are identified within the complex that are supported on the basis of large genetic distances, 100% bootstrap values, toxicity, and mating compatibility. Every isolate of three of the groups that has been tested is nontoxic, whereas every isolate of the remaining two groups is toxic. These phylogenetic groups were previously identified within the A. tamarense complex and given geographic designations that reflected the origins of known isolates. For at least two groups, the geographically based names are not indicative of the range occupied by members of each group. Therefore, we recommend a simple group-numbering scheme for use until the taxonomy of this group is reevaluated and new species are proposed.