Anthropogenic emissions of carbon dioxide (CO2) are rapidly increasing the atmospheric concentration of CO2 and reducing the pH of the surface ocean, a phenomenon referred to as ocean acidification. Ocean acidification could have negative effects on marine organisms. As a result of increasing seawater CO2 and decreasing pH, the availability of carbonate ions and the saturation state with respect to calcium carbonate minerals will decrease. As a consequence, the formation of calcareous hard-parts will become increasingly challenging for marine calcifying organisms such as corals, coralline algae, crustaceans, mussels, oysters and clams. This study provides information on the effects of increasing CO2 and decreasing pH on the growth rate of Isognomon alatus (flat-tree oyster) under controlled laboratory conditions as well as in a natural environment. In controlled laboratory experiments, I. alatus lost weight and experienced negative growth rates of –0.56 + 0.36 mg g−1day−1 under average pHtotal values of 7.8 compared to a loss of –0.26 + 0.23 mg g−1day−1 under average ambient pHtotal conditions of 8.1. In contrast, I. alatus incubated in a natural environment showed gain in weight and positive growth despite exposure to pH levels (~7.4) during low tide significantly lower than those experienced in the laboratory. These results suggest that rising CO2 and ocean acidification could have drastic negative consequences on I. alatus, but that additional factors (e.g., food availability) need to be considered in evaluating the response of this marine calcifier to these ongoing perturbations.