We have sequenced the four actin genes of Caenorhabditis elegans. These four genes encode typical invertebrate actins and are highly homologous, differing from each other by, at most, three amino acid residues. As a first step toward an understanding of the developmental regulation of this gene set we have also sequenced mutant actin genes. The mutant genes were cloned from two independent revertants of a single dominant actin mutant. For both revertants, reversion was accompanied by an actin gene rearrangement. The accumulation of actin mRNA during development in these two revertants is different from that of wild-type animals. We present here a correlation between actin gene structure and expression in wild-type and mutant animals. The results, suggest that co-ordinate regulation of actin genes is not essential for wild-type muscle function. In addition, it appears that changes in the 3' region of at least one of the actin mRNA may affect its steady-state regulation during development.