Using a cloned cDNA complementary to a portion of the mRNA for the 50 kd human epidermal keratin, we have screened a human genomic library and have isolated and sequenced the gene encoding this keratin. A comparison of the keratin gene with the very distantly related vimentin gene has enabled us to explore the relation between the evolutionary conservation of structure in intermediate filament (IF) subunits and the conservation of structure in IF genes. Our results reveal that not only the secondary structure of the IF proteins, but also the structural skeleton of their genes, has been maintained throughout evolution. These characteristics have persisted despite considerable flexibility in both protein and nucleic acid sequence. Surprisingly, although the positions of the introns within these two genes are highly conserved, they do not seem to correspond to the boundaries of the structural domains common to all IF subunits.