Ocean acidification, characterized by elevated pCO? and the associated decreases in seawater pH and calcium carbonate saturation state (?), has a variable impact on the growth and survival of marine invertebrates. Larval stages are thought to be particularly vulnerable to environmental stressors, and negative impacts of ocean acidification have been seen on fertilization as well as on embryonic, larval, and juvenile development and growth of bivalve molluscs. We investigated the effects of high CO? exposure (resulting in pH?=?7.39, ?(ar)?=?0.74) on the larvae of the bay scallop Argopecten irradians from 12 h to 7 d old, including a switch from high CO? to ambient CO? conditions (pH?=?7.93, ?(ar)?=?2.26) after 3 d, to assess the possibility of persistent effects of early exposure. The survival of larvae in the high CO? treatment was consistently lower than the survival of larvae in ambient conditions, and was already significantly lower at 1 d. Likewise, the shell length of larvae in the high CO? treatment was significantly smaller than larvae in the ambient conditions throughout the experiment and by 7 d, was reduced by 11.5%. This study also demonstrates that the size effects of short-term exposure to high CO? are still detectable after 7 d of larval development; the shells of larvae exposed to high CO? for the first 3 d of development and subsequently exposed to ambientCO? were not significantly different in size at 3 and 7 d than the shells of larvae exposed to high CO? throughout the experiment.