While ocean acidification (OA) is expected to have wide-ranging negative effects on marine species, organisms currently living in variable pH environments that expose them intermittently to pH values approaching those predicted for the future, may be better adapted to tolerate prolonged exposure to high pCO 2 levels caused by OA. Seasonal upwelling brings low pH water to the surface along the Pacific Coast of North America. In Monterey Bay, California Euphausia pacifica , a key species supporting a diverse multi-trophic-level ecosystem, currently experiences broad pCO 2 and pH ranges due to both diel vertical migrations and seasonal upwelling. We determined tolerances of E. pacifica to prolonged exposure to pH levels predicted for 2100 by maintaining adults at two pCO 2 levels (380 and 1200 µatm) for 2 months. Rates of survival and moulting were the same at both pCO 2 levels. High pCO 2 slowed growth in all size classes. In additional experiments to determine pCO 2 threshold levels above which E. pacifica is adversely affected, survival was not affected down to pH 6.96 (6050 µatm), but declined rapidly at pH 6.92 (7228 µatm) and lower, with 100% mortality within 10 d at pH 6.89.