A clone of the Drosophila melanogaster (Meigen) gene Act5C was used to isolate an actin gene from a Hessian fly, Mayetiola destructor (Say), genomic library in phage lambda. A combined molecular and cytological analysis with the Hessian fly actin gene (designated MdA1) was undertaken. The coding region of this gene was contiguous and encoded a protein that was 99% identical to the intersegmental muscle actins 57A and 87E from D. melanogaster. Additionally, the protein was >97% identical to the flight-muscle-specific actin 88E from D. melanogaster as well as muscle actins 1 and 2 from Bombyx mori (L.). Only the muscle actin 79B from D. melanogaster, a muscle actin in leg and thorax, showed <97% identity with actin 1 from Hessian fly. The actin 1 from Hessian fly shared less amino acid identity (94–95%) with the cytoplasmic actins from D. melanogaster, Anopheles gambiae (Giles), and B. mori, with differences occurring in a conserved region of the cytoplasmic actins proposed to function in interaction with actin binding proteins. These results are consistent with the Hessian fly MdA1 gene encoding a muscle actin. When compared with the D. melanogaster Act57A and Act87E genes, the Hessian fly MdA1 gene revealed 81% identity at the nucleotide level, with most of the variation associated with third-codon-position G C content. The MdA1 gene also had a high degree of general synonymous codon usage bias as measured by scaled chi-square. Southern gel blot analyses as well as in situ hybridization on salivary polytene chromosomes with MdA1 as the probe revealed that a gene family with at least five members encodes the actins in Hessian fly. Future identification of promoters for actins from Hessian fly should prove useful for gene expression in transgenic constructs.