The first confirmed death of a marine mammal due to domoic acid poisoning was reported in California, U.S.A. during May and June, 1998. This mortality of sea lions was determined to be caused by their ingestion of sardines and anchovies which had accumulated high levels of the neurotoxin. Sardines and anchovies which were harvested from nearshore waters between San Francisco Bay and Monterey Bay, California during a 3 week cruise in May showed the highest levels of domoic acid in Monterey Bay (up to 2300 ug DA/g viscera on 24 May), at the same time when sea lions with neurological symptoms stranded in the Bay. Four research cruises off the coasts of California and Oregon were conducted in June and July to determine the distribution of domoic acid-producing Pseudo- nitzschia. The pennate diatoms, Pseudo-nitzschia multiseries and P. australis were the dominant, toxin-producing phytoplankton constituting algal blooms near coastal locations where sea lions with neurological symptoms had stranded, with the highest cellular toxin levels at 6 pg/cell (P. multiseries) and 78 pg/cell (P. australis). P. australis was the dominant Pseudo-nitzschia species in nearshore waters off the central Oregon coast during July, located within 20 km of the coast at a time when domoic acid in razor clams was reported in that same area. Hydrographic data and satellite imagery at the time of these cruises indicated that the greatest numbers of toxic cells were positioned in water masses associated with upwelling zones. Nutrient levels at these sites in California and Oregon were less than those typically measured during periods of active upwelling due to the 1998 El Niûo event. These results agree with long- term studies of phytoplankton succession (Chavez et al., in press) which indicate that pennate diatoms dominate during post-upwelling conditions when nutrient levels are less than maximal.