The balance between photosynthesis and photorespiration in many species, including most crop plants, is determined by the kinetic properties of ribulose-l,5-bisphosphate (RuBP) carboxylase/oxygenase1,2. Photosynthesis is initiated by the carboxylase activity3 while the oxygenase activity catalyses the first reaction in the photorespiratory pathway1,2,4. In these reactions, CO2 and O2 are competitive substrates2. Because O2 inhibits carboxylation, and photorespiration oxidizes reduced carbon to CO2 with no known benefit to the plant, it has been suggested that photosynthetic efficiency and thus productivity might be increased by chemical or genetic alterations of the enzyme which increase carboxylation or decrease oxygenation5-7. On the other hand, it has been argued that RuBP carboxylase/oxygenase cannot completely discriminate between CO2 and O2, so that photorespiration is unavoidable8. From analyses of RuBP carboxylase/oxygenase enzymes purified from several different species, we report here large differences in specificity towards the substrates CO2 and O2. Evolutionary pressures seem to have directed the enzyme towards more efficient utilization of CO2.