Ribulose-1, 5-bisphosphate carboxylase/oxygenase(Rubisco) was highly purified from nine species native to the Balearic Islands. The specificity factor for Rubisco from each species was determined and compared with that of wheat. The specificity factors were comparable with published values for C3 plants, how-ever, the values for three species exceeded those of wheat. Natural selection in the hot, dry mediterranean conditions of the Balearic Island Mallorca has slightly increased the specificity factor of Rubisco in some species.