Starch gel electrophoresis of creatine kinase (CK) isozymes of Xenopus tropicalis shows that at least two different genes code for CK in this diploid (2n = 20) species. These genes seen to be orthologous to the CK-A and CK-C genes of extant crossopterygian fish. Additional isozymes may be interpreted either as products of duplicate genes or, more probably, as epigenetically modified forms of the homodimers AtAt and CtCt, respectively. The originally tetraploid species X. laevis laevis (2n = 36), which may have arisen by hybridization of diploid ancestors some 30-40 million years ago, has retained expression of all duplicate CK-A and CK-C genes. Differential expression during ontogenesis (CK-A genes) and in different adult tissues (CK-C genes) indicates that divergence occurred not only with respect to the primary sequence of these duplicate genes, but also with respect to the regulation of their expression. In the interspecific hybrid X. l. laevis x X. tropicalis, all parental CK genes appear to be expressed simultaneously in the heart. However, several subunit combinations cannot be detected on the zymograms.