We examined the growth rate (μ) of Ulva lactuca L. (collected from Roskilde Fjord, Denmark in 1987) at different levels of dissolved inorganic carbon (DIC), pH and oxygen in two culture facilities. Growth was faster in Facility A (μmax ca 0.3 d-1) than in B (μmax ca 0.2 d-1), probably because of more efficient stirring and higher light intensity. The growth-DIC response curve exhibited low half-saturation constant (K1/2) values (0.35 m M DIC in A, 0.55 m M in B) and growth rates close to μmax at natural seawater concentration of 2 m M DIC. Growth rate showed a low sensitivity to oxygen over a wide range of DIC and oxygen concentrations. Collectively, the results demonstrated an efficient mechanism for DIC use, unaffected by acclimatization to DIC concentrations between 0.2 and 3 m M. The growth rate decreased little between pH 7.5 and 9 at 2 m M DIC, but steeply above pH 9 approaching zero just above pH 10. The decline of growth at high pH may result from direct pH effects on cell pH, reduced HCO3- availability and impaired operation of the carbon uptake process. The growth responses of U. lactuca to DIC, pH and oxygen resembled those observed in previous short-term photosynthetic experiments. This similarity is probably due to the fast growth of U. lactuca which means that photosynthetic products are rapidly converted into cell growth. Based on the culture experiments we argue that field plants of U. lactuca not exposed to stagnant water and DIC depletion are likely to be limited in growth by environmental factors other than DIC (e.g. light and nutrients). Dense mats of U. lactuca, however, may show reduced growth as a result of DIC depletion, high pH and self-shading. © 1990 Springer-Verlag.