The nucleotide sequence of a cDNA corresponding to the transcription product of the mitochondrial structural gene cytochrome c oxidase subunit I was determined. A polypeptide of 508 residues was deduced from the reading frame established by the nucleotide sequence. TGA codes tryptophan, as in most other mitochondrial systems. From the comparison of the amino acid sequence of the putative Blattella germanica cytochrome c oxidase with those of Drosophila, we conclude that ATA and AGA codons specify methionine and serine respectively, instead of isoleucine and arginine. The sequence proposed for cytochrome c oxidase subunit I of B. germanica is largely homologous to that of other species. From the alignment of cytochrome c oxidase subunit I protein sequences we have found that 125 residues (positional identity of 22.3%) have remained invariant in this enzyme for more than one billion years of divergence. There is a developmental pattern of gene expression affecting the embryo stages. Northern blot analysis of RNA samples from different adult female tissues shows high cytochrome c oxidase subunit I mRNA levels in gut and fat body, and lower levels in ovary and colleterial glands.