Dihydroisoxazole derivatives are conveniently converted to .gamma.-hydroxyamino acid derivatives which are important as various synthetic intermediates by a catalytic hydrogenation reaction. High-purity monatins which may be used as sweeteners or ingredients thereof can be obtained by subjecting a 5-indolylmethyl-4,5-dihydroisoxazole-3,5-dicarboxylic acid to catalytic hydrogenation.