The cosmopolitan dinoflagellate genus Alexandrium, and especially the A. tamarense species complex, contain both toxic and non-toxic strains. An understanding of their evolution and paleogeography is a necessary precursor to unravelling the development and spread of toxic forms. The inclusion of more strains into the existing phylogenetic trees of the Alexandrium tamarense species complex from LSU rDNA sequences has confirmed that geographic distribution is consistent with the molecular clades, but not with the three morphologically-defined species that constitute the complex. In addition, a new clade has been discovered, representing Mediterranean non-toxic strains. The dinoflagellates fossil record was used to calibrate a molecular clock: key dates used in this calibration are the origins of the Peridiniales (estimated at 190 Ma), Gonyaulacaceae (180 Ma) and Ceratiaceae (145 Ma). Based on the data set analyzed, the origin of the genus Alexandrium was estimated to be around Late Cretaceous (77 Ma) with its earliest possible origination in the mid Cretaceous (119 Ma). The A. tamarense species complex potentially diverged around the early Neogene (23 Ma), with a possible first appearance in the late Paleogene (45 Ma). A paleobiogeographic scenario for Alexandrium is based on: 1) the calculated possible ages of origination for the genus and its constituent groups; 2) paleogeographic events determined by plate movements, changing ocean configurations and currents, as well as climatic fluctuations, and 3) the present geographic distribution of the various clades of the Alexandrium tamarense species complex.Key index words: Alexandrium tamarense/Alexandrium catenella/Alexandrium fundyense species complex, biogeography, dinocysts, dinoflagellates, evolution, harmful algal blooms, molecular clock, phylogeny, toxic algae.