The effect of elevated pH on growth and on production of the neurotoxin domoic acid was studied in selected diatoms belonging to the genera Pseudo-nitzschia and Nitzschia. Growth of most of the 11 species studied stopped at pH values of 8.7 to 9.1. However, for P. delicatissima and N. navis-varingica the pH limit for growth was higher, 9.3 and 9.7 to 9.8, respectively. A compilation of all available data on the pH limits for growth of marine planktonic diatoms suggests that species from ponds and rock pools all have higher limits than coastal and oceanic species. Taking only coastal and oceanic species into account, the data suggest that smaller species have a higher upper pH limit for growth than larger species. Elevated pH induced production of domoic acid in P, multiseries in amounts comparable to those detected previously under silicate and phosphate limitation. As Pseudo-nitzschia species are found in high concentrations in nutrient- enriched areas, high pH and hence induction of the production of domoic acid would be expected during blooms. These results may help to understand when and why Pseudo-nitzschia species produce domoic acid in the field.