Ph.D. Thesis submitted to the National University of Ireland, Galway. The objectives of this study were to: 1. Determine which Pseudo-nitzschia species are present in Irish waters using light microscopy, scanning and transmission electron microscopy; 2. Investigate the hydrographic conditions that relate to the distribution of the genus Pseudo-nitzschia; 3. Determine which species are capable of producing domoic acid in unialgal cultures; 4. Compare SSU and ITS1 and part of the 5.8S ribosomal DNA sequences of Irish Pseudo-nitzschia isolates with available sequences of Pseudo-nitzschia isolates from other geographic regions; 5. Examine the efficacy of existing oligonucleotide probes to aid identification of Pseudo-nitzschia strains in Irish waters.