The anuran amphibian, Rana catesbiana, has been found to possess at least two kinds of immunoglobulins corresponding to gammaG- and gammaM-classes. These classes have the same chain structures as those of their counterparts in higher animal species. Light chains of both immunoglobulins had molecular weights of 20,000. Heavy chains of the gammaM-class had molecular weights of 72,100; those of the gammaG-class had molecular weights of 53,600. The carbohydrate content of the gammaG-immunoglobulin was 2.1%, and that of the gammaM-protein was 10.8%. The amino acid compositions of the immunoglobulins were generally similar to those of mammalian immunoglobulins. After a single injection of phage antigen (f2), the order of appearance of phage-neutralizing activity in the frog immunoglobulin classes was (a) gammaM-antibodies, and (b) gammaG-antibodies. The results of this and previous studies suggest that the gammaG-immunoglobulins emerged at some point in evolution between the elasmobranchs and the anuran amphibians.