We have determined the complete nucleotide sequences of two chicken beta-tubulin genes, beta 1 and beta 2. These genes display an unusual pattern of segmental homology which indicates that they originally arose by gene duplication and have subsequently coevolved by a process that included localized gene conversion or intergenic recombination. Since the beta-tubulin polypeptides encoded by the two genes are virtually identical (99.5%), particularly in the major beta-tubulin isotype defining regions, they almost certainly constitute a single isotypic class of beta tubulin. However, the regulatory properties of the two genes are highly divergent as indicated by analysis of their patterns of expression in different chicken cell types. beta 1 is the major transcript detected in skeletal muscle myoblasts, whereas beta 2 is the major beta-tubulin transcript in cultured sympathetic neurons. The existence of these two genes appears to derive from a regulatory requirement whereby the expression of a single tubulin isotype is mediated through different regulatory programs in development and differentiation. These results thus provide direct experimental support for the hypothesis that gene conversion and intergenic recombination play an important role in evolution by uncoupling the evolution of structural genes from the regulatory sequences which control them.