Flies and moths are approximately as distant phylogenetically as are mammals and birds. In terms of morphology, physiology and biochemistry, the complex proteinaceous eggshell or chorion differs substantially in these two insect groups, which are typified by Drosophila melanogaster and Bombyx mori. The major chorion proteins of moths are encoded by two families of genes, A and B, which have no obvious homologues in flies. Unlike Drosophila, where chorion genes are oriented in tandem, moths show mostly chorion gene pairs (A plus B) that are divergently transcribed and coordinately expressed. The 5' ends of the paired genes are separated by a DNA segment of only 300 +/- 50 base pairs, which may well include at least some of the cis-regulatory elements necessary for gene expression. Despite these differences, we have tested whether moth chorion genes might be expressed in flies. Cloned DNA fragments bearing moth chorion genes were introduced into the Drosophila germ line by P-element-mediated transformation. Analysis of RNAs from transformed lines revealed that the genes are expressed with correct sex, tissue and temporal specificity, resulting in the accumulation of abundant moth chorion transcripts in late fly follicles.