The catalytic subunit, gamma, of phosphorylase kinase contains two calmodulin-binding sequences that define a domain in gamma that is homologous to the troponin-C-binding domain in troponin I. The homology is based on both sequence and functional similarities. To account for this homology, it has been proposed that the calmodulin-binding sequences in gamma and the troponin-C-binding domain in troponin I have evolved from a common ancestor. We investigated this possibility by comparing the exon structure of the gamma gene with that of troponin-I gene over their homologous domains. In the quail troponin-I gene, it is known that the entire troponin-C-binding domain is encoded by a single exon. However, two exons are found to encode the calmodulin-binding domain in the gamma gene from mouse. This result indicates that convergent evolution may be responsible for the sequence and functional similarities between the homologous domains in troponin I and gamma.