Allantoinase and allantoicase were co-purified from frog (Rana catesbeiana) liver. The ratio of the two enzyme activities remained constant during purification and was unchanged by a variety of treatments of the purified enzyme. These results suggest that allantoinase and allantoicase are located in the same protein. It was found that the two hepatic enzyme activities are also associated with the same protein in other frogs (Xenopus laevis and Rana nigromacultata), tadpoles (R. catesbeiana), and newts (Triturus pyrrhogaster). In contrast, allantoinase and allantoicase were found to be different proteins in marine fish and invertebrate liver.