species of amphibians has been discovered during recent years. In the case of Leptodactylus, a very puzzling problem in the L. ocellatus complex has been pointed out by one of us (Cei 1948, 1949, 1950). Among the neotropical forms of the L. pachypus stock (or subgenus), we can consider at first 2 groups of frogs: the ocellatus-bolivianus group and the pentadactylus group, both widely distributed in the cis-Andine continental belt. In addition to many distinctive morphological features, differences in some physiological or metabolic characters of these groups have recently been found, e.g., the presence in the skin of different, probably opposite, enzymatic systems for bioamine synthesis leading to phenylalkylamines (Leptodactyline) in the ocellatus-bolivianus group, and to indolalkylamines (5-H-triptamine and derivatives) in the pentadactylus group (Erspamer and Cei 1963). But in the ocellatus-bolivianus group, the widespread ocellatus complex, formerly a Linnean taxon (Leptodactylus ocellatus = Rana ocellata Linne), includes 2 closely related, partly sympatric species which can be distinguished by many physiological characters but by few morphological ones. There are, for example, striking differences in the male sex cycle and sex character regulation (Cei 1948, 1949, 1950, 1962; Cohen 1962), a remarkable difference in the electrophoretic properties of the hemoglobins (Bertini and Rathe 1962), considerable chromatographic difference in the Leptodactyline content of the skin (Cei and Erspamer 1963) and a strikingly different populational distribution of the seroprotein electrophoretic patterns (Cei and Bertini 1961, Gallopin 1962). These sibling species, Leptodactylus chaquensis Cei and L. ocellatus (Linne), are found in well-defined areas, overlapping in more or less extensive zones from northeastern Brazil to the Parana River (Santa Fe, Argentina). It appeared worthwhile applying to these sibling species of the ocellatus complex the criterion of serological affinity, as indicated by precipitin tests. In the present paper, we report on such tests of serum samples of Leptodactylus ocellatus and L. chaquensis populations in comparison with those of the more distantly related L. pentadactylus. It is a pleasure to acknowledge the courtesy of Dr. Alan Boyden of Rutgers University. We are greatly indebted to him for reviewing this article.