With the growth of aquaculture, toxic phytoplankton have become a significant problem. At present, the identification of specific algal species within blooms requires considerable expertise, and the methods are both costly and time consuming. It is clear that alternative diagnostic tests must be developed that allow rapid and simple assessments to be made on site. We report on the identification of unique sequences within the SSU and LSU rDNA genes of Alexandrium species that provide data for the design of species- and genus-specific molecular probes. In addition we examine some aspects of the phylogeny of Alexandrium isolates from New Zealand waters.