The evolutionary homology of pepsinogens was further evaluated by isolating and characterizing the pepsinogen of the esophageal glands of Rana catesbeiana. Like other pepsinogens, this esophageal enzyme was activated by acid; the resulting pepsin was optimally active between pH 1.4 and 2.0, and was irreversibly denatured above pH 7.0. Chromatography on DEAE-cellulose at pH 7.0 separated four acid protease fractions corresponding to pepsinogens B, D, A, and C. Hydroxylapatite chromatography of the major peptic fraction, pepsinogen A, followed by rechromatography on DEAE-cellulose at pH 8.5 yielded pure pepsinogen A which was free of detectable contaminants. Estimation of molecular weight by gel filtration on Sephadex G-75, by polyacrylamide gel electrophoresis in 0.1% SDS and by sedimentation equilibrium gave values of 31,500, 33,500, and 33,700, respectively. These studies suggest that the difference between bullfrog and other pepsinogens is located in a 90 to 110 amino acid (Mr 9,000 to 11,000) region of the molecule which must be remote from the catalytic and immunogenic sites. This lower molecular weight pepsinogen should thus provide a simpler molecular model for study of the catalytic and immunogenic properties. The pepsinogen from bullfrog gastric mucosa was found to have similar properties which suggested that gastric and esophageal pepsinogens of bullfrog are derived from a common ancestral origin. This archetypic pepsinogen may have undergone deletions late in evolution to render modern bullfrog pepsinogen structurally dissimilar from other described pepsinogens. The modern bullfrog enzyme has retained peptic enzymatic activity despite these evolutionary changes.