To gain an understanding of the species of Pseudo nitzschia that are the major toxin producers in the Pacific Northwest, representative isolates from the Ecology and Oceanography of Harmful Algal Blooms in the Pacific Northwest (ECOHAB-PNW) study site were grown as clonal cultures.  Both particulate and dissolved domoic acid (DA) were measured at various stages of growth using a receptor-binding assay and an enzyme-linked immunosorbent assay respectively, and compared to toxin data obtained during a cruise in September 2003.  A clonal P. multiseries culture isolated from a northern beach site contained the highest particulate DA (pDA) level of 70.4nmol l⁻¹ and released the highest amount of dissolved DA (dDA, >5nmol l⁻¹), illustrating the potential for this species to cause harm.  A clonal P. australis isolate also produced relatively high levels of pDA (3.2nmol l⁻¹) and dDA (4.3nmol l⁻¹), whereas P. delicatissima, P. cf. pseudodelicatissima, P. pungens, and P. fraudulenta produced low or undetectable levels of pDA and dDA.  These culture studies suggest that the dDA levels of up to 17.6nmol l⁻¹ measured during September 2003 were owing to toxin produced and released primarily by P. australis cells.  It is estimated that the maximum specific toxicity reached by P. australis at a toxic 'hot spot' off the Washington coast in mid-September 2003 was 94.4pg cell⁻¹.