The marine diatom Skeletonema costatum was grown at different pH values in a semicontinuous cultivation system which kept the pH at a pre-set value. Growth rate was nearly constant at pH 6·5–8·5 (average 2·4 divisions day−1) and declined at pH > 9. Organic carbon (C) production decreased from an average of 5·3 mg l−1 at pH 6·5–8·5 to 2 and 1 mg l−1 at pH 9·0 and 9·4, respectively. Storage β-1,3-linked glucan decreased from a high of 7·1 mg l−1 at pH 6·5 to 0·2 mg l−1 at pH 9·4, and, concomitantly, the percentage of total organic C as glucan C decreased from 60% to 10%. Cellular concentration of amino acids varied from about 62 to 8 mM over the pH range 8·0–9·4. At pH 9·4 the concentration of glutamine was below the detection limit. The extracellular production of carbohydrates in the pH range 6·5–8·5 was only 3·7% (average) of total C production and increased at pH 9·0 and 9·4. The decrease in growth rate at pH > 9·0 might be caused by a decrease in the rate of some important biochemical reactions, as well as by a change in cell membrane properties in this pH range.