The marine dinoflagellate Alexandrium tamarense is a globally distributed harmful algae known to be the cause of Paralytic Shellfish Poisoning (PSP). The toxin produced by Alexandrium which is the cause of this syndrome is known as ‘saxitoxin’. Saxitoxin is also produced by some fresh water cyanobacteria. This thesis was designed to investigate about the biosynthesis of saxitoxin in dinoflagellates and the nature and phylogeny of the biosynthesis genes, based upon the knowledge from fresh water cyanobacteria. In this study, putative models for six genes involved in biosynthesis of saxitoxin were recovered. The gene models formulated fitted the toxin profile of Alexandrium tamarense strain A5. Further, an attempt was made to obtain full length sequences for four of these genes by the 5’ and 3’ RACE PCR technique and by filling in the gaps between sequences. A phylogenetic analysis was carried out for five of these genes using the Maximum Likelihood approach in order to obtain some insight into the phylogeny, and the relation of the saxitoxin genes in dinflagellates to the saxitoxin clusters in cyanobacteria. The Alexandrium gene models did not show a high sequence similarity to homologous genes from cyanobacteria and all of the genes always clustered together to form a separate dinoflagellate clade, distinct from the well supported clade of the toxic genes from cyanobacteria and the other related protein sequences. The dinoflagellate gene models showed a similarity in the predicted functional domain with the cyanobacterial toxin genes, but no further similarity was observed. It is difficult to comment about the phylogeny of the dinoflagallate genes. The dinoflagellates might have obtained the genes from cyanobacteria through an ancient Horizontal Gene Transfer (HGT) event, or the from an ancient common ancestor, or the genes in cyanobacteria and dinoflagellates might have evolved independently as a result of convergent evolution.