Increased levels of atmospheric CO2 are anticipated to cause decreased seawater pH. Despite the fact that calcified marine invertebrates are particularly susceptible to acidification, barnacles have received little attention. We examined larval condition, cyprid size, cyprid attachment and metamorphosis, juvenile to adult growth, shell calcium carbonate content, and shell resistance to dislodgement and penetration in the barnacle Amphibalanus amphitrite reared from nauplii in either ambient pH 8.2 seawater or under CO2-driven acidification of seawater down to a pH of 7.4. There were no effects of reduced pH on larval condition, cyprid size, cyprid attachment and metamorphosis, juvenile to adult growth, or egg production. Nonetheless, barnacles exposed to pH 7.4 seawater displayed a trend of larger basal shell diameters during growth, suggestive of compensatory calcification. Furthermore, greater force was required to cause shell breakage of adults raised at pH 7.4, indicating that the lower, active growth regions of the wall shells had become more heavily calcified. Ash contents (predominately calcium carbonate) of basal shell plates confirmed that increased calcification had occurred in shells of individuals reared at pH 7.4. Despite enhanced calcification, pen-etrometry revealed that the central shell wall plates required significantly less force to penetrate than those of individuals raised at pH 8.2. Thus, dissolution rapidly weakens wall shells as they grow. The ramifications of our observations at the population level are important, as barnacles with weakened wall shells are more vulnerable to predators. © Inter-Research 2009.