Ocean acidification has severe implications for marine ecosystems. Tropical coral reef communities are particularly vulnerable as they are dominated by marine calcifiers such as corals and crustose coralline algae (CCA). CCA are a fundamental component of reef communities in that they cement reef fragments together, provide critical settlement cues for coral larvae, build reef caps (algal ridges), and are a source of primary production. Despite the key role of CCA in tropical communities, little is known about species- specific physiological responses to ocean acidification. In order to elucidate these processes, experiments were conducted in Moorea, French Polynesia that measured photosynthesis, respiration, and calcification of CCA in response to incubation in seawater with ambient and elevated levels of pCO₂. One of the dominant species of CCA in the Moorea lagoon is Hydrolithon onkodes. Circular cores of H. onkodes were collected from the reef crest and exposed for 6 weeks to two seawater treatments: ambient pCO₂ (~380 ppm) and elevated pCO₂ (~800 ppm). The elevated pCO₂ (ocean acidification) treatment was established by directly bubbling carbon dioxide gas at a concentration of 800 ppm into treatment aquaria, and represents the oceanic conditions expected by the year 2100 in business-as-usual carbon emission scenarios. Calcification rates of H. onkodes were determined by measuring the change in mass by buoyant weighing and by the alkalinity anomaly technique during short incubations in a sealed metabolic chamber. Photosynthesis and respiration were determined by measuring the change in oxygen concentration during incubations in a sealed metabolic chamber in light and dark conditions, respectively. Elevated pCO₂ significantly increased respiration, and had no effect on photosynthesis or calcification in H. onkodes after 6 weeks of incubation. These results contrast recent studies that emphasize the negative effects of elevated pCO₂ on temperate and free-living CCA, and indicate that effects of ocean acidification on CCA may vary between species.