Shelled pteropods are planktonic molluscs that may be affected by ocean acidification. Limacina retroversa from the Gulf of Maine were used to investigate the impact of elevated carbon dioxide (CO2) on shell condition as well as swimming and sinking behaviours. Limacina retroversa were maintained at either ambient (ca. 400 ?atm) or two levels of elevated CO2 (800 and 1200 ?atm) for up to four weeks, and then examined for changes in shell transparency, sinking speed, and swimming behaviour assessed through a variety of metrics (e.g., speed, path tortuosity, wing beat frequency). After exposures to elevated CO2 for as little as four days, the pteropod shells were significantly darker and more opaque in the elevated CO2 treatments. Sinking speeds were significantly slower for pteropods exposed to medium and high CO2 in comparison to the ambient treatment. Swimming behaviour showed less clear patterns of response to treatment and duration of exposure, but overall, swimming did not appear to be hindered under elevated CO2. Sinking is used by L. retroversa for predator evasion, and altered speeds and increased visibility could increase the susceptibility of pteropods to predation.