Paralytic Shellfish Poisoning (PSP) is a problem worldwide and in many cases is linked to the occurrence of toxic algal cells of the genus Alexandrium (Dinophyceae) in the marine environment. In the UK the species which has been linked with PSP is A. tamarense. Both toxic and non-toxic strains of this species have been found in the UK but these strains cannot be differentiated morphologically. The ability of certain species of Alexandrium (including A. tamarense ) to produce toxins appears to correlate with phylogenetic lineage. North American and temperate Asian lineages consist exclusively of toxic strains and the western European line consists solely of non-toxic strains. To date, all toxic strains of Alexandrium found in UK waters can be assigned to the North American lineage based on their rRNA gene sequences, and it has been suggested that these strains are introductions, whilst the non-toxic strains are native to UK waters. We have conducted studies to determine if the correlation between toxicity and genetic lineage holds true for Alexandrium strains isolated in UK waters. Sequence and toxicity data for A. tamarense isolates will be presented.