Ocean acidification results from an increase in the concentrations of atmospheric carbon dioxide (CO2) impacts on marine calcifying species, which is predicted to become more pronounced in the future. By the end of this century, atmospheric pCO(2) levels will have doubled relative to the pre-industrial levels of 280ppm. However, the effects of pre-industrial pCO(2) levels on marine organisms remain largely unknown. In this study, we investigated the effects of pre-industrial pCO(2) conditions on the size of the pluteus larvae of sea urchins, which are known to be vulnerable to ocean acidification. The larval size of Hemicentrotus pulcherrimus significantly increased when reared at pre-industrial pCO(2) level relative to the present one, and the size of Anthocidaris crassispina larvae decreased as the pCO(2) levels increased from the pre-industrial level to the near future ones after 3days' exposure. In this study, it is suggested that echinoid larvae responded to pre-industrial pCO(2) levels. Ocean acidification may be affecting some sensitive marine calcifiers even at the present pCO(2) level.