Information on the effects of elevated ammonia on invertebrates in general, and polar Mollusks in particular, is scant. Questions of ammonia sensitivity are interesting for several reasons, particularly since predicted global change scenarios include increasing anthropogenic nitrogen and toxic ammonia. Furthermore, polar zooplankton species are often lipid-rich, and authors have speculated that there is a linkage between elevated levels of lipids/trimethylamine oxide and enhanced ammonia tolerance. In the present study, we sought to examine ammonia tolerance and effects of elevated exogenous ammonia on several key aspects of the physiology and biochemistry of the pteropod mollusk, Clione limacina antarctica. We determined that the 96-h LC50 value for this species is 7.465 mM total ammonia (Upper 95% CL = 8.498 mM and Lower 95% CL = 6.557 mM) or 0.51 mg/L as unionized ammonia (NH3) (at a pH of 7.756). While comparative data for mollusks are limited, this value is at the lower end of reported values for other species. When the effects of lower ammonia concentrations (0.07 mM total ammonia) on oxygen consumption and ammonia excretion rates were examined, no effects were noted. However, total ammonia levels as low as 0.1 mM (or 0.007 mg/l NH3) elevated the activity of the ammonia detoxification enzyme glutamine synthetase by approximately 1.5-fold. The values for LC50 and observable effects on biochemistry for this one species are very close to permissible marine ammonia concentrations, indicating a need to more broadly determine the sensitivity of zooplankton to potential elevated ammonia levels in polar regions.