The purpose of this study was to compare investment in structures for gas exchange in common carp, Cyprinus carpio, with different stress coping styles, which are known to differ in resting metabolic rate. Common carp were classified as proactive or reactive on the basis of rate of emergence from cover into a novel, potentially dangerous environment in three successive tests. The fish were then killed and their gill arches removed. Length-independent estimates of the size of the respiratory structures were derived by multivariate analysis of arch length and the number and length of gill filaments and lamellae for all gill arches. Overall gill area was also estimated. Filaments from the second gill arch were sampled for histological estimation of the extent to which the respiratory surface is covered by epithelial cells (hyperplasia). Proactive carp had longer gill filaments and more, longer lamellae, and consequently a larger gill surface area, than reactive carp. In contrast, the extent of hyperplasia was higher in reactive than proactive carp. Thus, compared to reactive fish, proactive carp had a larger respiratory surface, more of which was exposed to the surrounding water rather than being covered by epithelial cells. We suggest that the higher metabolic rate of proactive fish requires greater investment in, and greater exposure of, the respiratory structures. This is likely to make oxygen uptake more effective, but may also impose hidden costs of a proactive, aggressive lifestyle.