The transport of amino acids through isolated potato periderm and pear fruit cuticular membranes were investigated. The transport rate depended on the molecular size of amino acids and decreased with an increase in molecular size through extension of the hydrophobic -CH2- group. The selectivity coefficients of amino acids for the ammonium ion form of periderm and cuticular membranes were also determined, using the mass action law. They were found to be correlated with various physicochemical parameters such as the partial molar volume and the molecular weight.