The effects of high pH and low DIC (dissolved inorganic carbon) concentrations on growth rates of the dinoflagellate Ceratium were studied using 2 different approaches: (1) pH drift experiments, in which both pH and DIC change during the experiment, and (2) constant pH and DIC experiments at a range of fixed pH and DIC levels. The upper pH limits for growth were between 8.7 and 9.1 for the 3 species tested (C. furca, C. tripos and C. fusus) and thus within an environmentally relevant pH range. Hence, for Ceratium spp. and other dinoflagellates with upper pH tolerances for growth below or around 9, high pH will most likely periodically be a limiting factor in coastal areas. Different upper pH limits for growth were found using either the pH drift or the constant pH methods for the larger Ceratium species, but, for the smaller species, the 2 methods provided similar results. The results therefore suggest a combined pH/DIC limitation for large species such as C. tripos and C. furca in natural waters.