A sample set generated from field investigations of phytoplankton in Irish coastal and shelf waters carried out since 1993, was used to determine the temporal and spatial variability of species within the genus Pseudo-nitzschia. The study was carried out to ascertain the threat posed by this diatom to the Irish aquaculture industry. Six Pseudo-nitzschia spp. were positively confirmed using electron microscopy (P. pungens, P. multiseries, P. fraudulenta, P. australis, P. delicatissima and P. pseudodelicatissima). Pseudo-nitzschia seriata f. seriata, P. seriata f. obtusa, and P. subpacifica were tentatively identified and a further unidentified Pseudo-nitzschia species was also observed. One species (P. australis) isolated from Irish waters produced domoic acid (DA) under laboratory conditions. A further six of the species present have been reported as DA producers in the literature. There was no apparent relationship between species dominance with either season or hydrography. Pseudo-nitzschia pungens (June 1997 and August 1993) P. pseudodelicatissima (July, 1996) P. australis (September 1996), P. delicatissima (May 1996 and September 1996) and P. fraudulenta (October 1997) were at varying times dominant, either within the genus or the total phytoplankton population, in cell concentrations of up to 2 000 000 cells per litre.