ABSTRACT: Increased carbon dioxide (CO₂) concentration in the atmosphere will change the balance of the components of carbonate chemistry and reduce the pH at the ocean surface. Here, we report the effects of increased CO₂concentration on the early development of the sea urchins Hemicentrotus pulcherrimus and Echinometra mathaei. We examined the fertilization, early cleavage, and pluteus larval stage to evaluate the impact of elevated CO₂concentration on fertilization rate, cleavage rate, developmental speed, and pluteus larval morphology. Furthermore, we compared the effects of CO₂ and HCl at the same pH in an attempt to elucidate any differences between the two. Wefound that fertilization rate, cleavage rate, developmental speed, and pluteus larval size all tended to decrease with increasing CO₂ concentration. Furthermore, CO₂-seawater had a more severe effect than HCl-seawater on thefertilization rate. By contrast, the effects on cleavage rate, developmental speed, and pluteus larval morphology were similar for CO₂- and HCl-seawater. Our results suggest that both decreased pH and altered carbonate chemistry affect theearly development and life history of marine animals, implying that increased seawater CO₂ concentration will seriously alter marine ecosystems. The effects of CO₂ itself on marine organisms therefore requires further clarification.