The relationship among cellular levels of toxin, Pseudo-nitzschia distributions, and the energetic and highly variable coastal water masses of an upwelling- dominated region are explored using data collected during two summer cruises off the Washington coast in 1997 and 1998. During both years, an area of maximum domoic acid production was located approximately 50 km off the coast and the stations with the highest domoic acid levels corresponded to high numbers of Pseudo-nitzschia pseudodelicatissima (up to 1 million cells/L, 2.5 ug DA/L in 1997 and 200,000 cells/L, 0.2 ug DA/L in 1998), a species confirmed by receptor binding analysis and mass spectroscopy to produce toxin in this region. Other Pseudo-nitzschia species were present, but were always less than 5% of the total population when domoic acid was measured. In both years, Pseudo-nitzschia cells and domoic acid were observed from 0 to 40 m depth, although the highest levels of cells and toxin were in the upper 20 meters. In 1998, monthly cruises in the summer and early fall indicated that high levels of domoic acid in seawater covered an approximately 100 km2 area of the Washington coast and persisted at least until 1 October. The appearance of high levels of domoic acid (up to 2.6 ug DA/L), coincident with high numbers of P. pseudodelicatissima (up to 15 million cells/L) at Kalaloch beach on the central coast in late September, was followed by the accumulation of record levels of toxin in razor clams (287 ug DA/g) approximately one week later. This dramatic increase in toxin-producing cells on the coast immediately followed a strong upwelling event, indicated by high levels of silicate and nitrate in the absence of significant rainfall and hence river flow.