Changes in seawater carbonate chemistry that accompany ongoing ocean acidification have been found to affect calcification processes in many marine invertebrates. In contrast to the response of most invertebrates, calcification rates increase in the cephalopod Sepia officinalis during long-term exposure to elevated seawater pCO2. The present trial investigated structural changes in the cuttlebones of S. officinalis calcified during 6 weeks of exposure to 615 Pa CO2. Cuttlebone mass increased sevenfold over the course of the growth trail, reaching a mean value of 0.71 ± 0.15 g. Depending on cuttlefish size (mantle lengths 44-56 mm), cuttlebones of CO2-incubated individuals accreted 22-55% more CaCO3 compared to controls at 64 Pa CO2. However, the height of the CO2-exposed cuttlebones was reduced. A decrease in spacing of the cuttlebone lamellae, from 384 ± 26 to 195 ± 38 μm, accounted for the height reduction The greater CaCO3 content of the CO2-incubated cuttlebones can be attributed to an increase in thickness of the lamellar and pillar walls. Particularly, pillar thickness increased from 2.6 ± 0.6 to 4.9 ± 2.2 μm. Interestingly, the incorporation of non-acid-soluble organic matrix (chitin) in the cuttlebones of CO2-exposed individuals was reduced by 30% on average. The apparent robustness of calcification processes in S. officinalis, and other powerful ion regulators such as decapod cructaceans, during exposure to elevated pCO2 is predicated to be closely connected to the increased extracellular [HCO3-] maintained by these organisms to compensate extracellular pH. The potential negative impact of increased calcification in the cuttlebone of S. officinalis is discussed with regard to its function as a lightweight and highly porous buoyancy regulation device. Further studies working with lower seawater pCO2 values are necessary to evaluate if the observed phenomenon is of ecological relevance. © 2010 Springer-Verlag.