The complete amino acid sequence of the Drosophila melanogaster Cu,Zn Superoxide dismutase subunit has been determined by automated Edman degradation. Sequence analyses were performed on the intact S-carboxymethylated protein, two fragments derived from CNBr cleavage, and three peptides recovered from mouse submaxillary protease digestion of the reduced and S-carboxymethylated enzyme. The peptides were aligned by characterizing peptides yielded by trypsin and Staphylococcus aureus V8 protease. All the peptides studied were purified exclusively by reverse-phase columns of HPLC and were analyzed with an improved liquid-phase sequencer. A molecular weight of 15,750 (subunit) was calculated from the 151 residues sequenced. The amino acid sequence of the Drosophila Superoxide dismutase subunit is compared with that of four other eucaryotes: man, horse, cow, and yeast. Comparison of the five primary structures reveals very different rates of evolution at different times. Copper-zinc superoxide dismutase appears to be a very erratic evolutionary clock. ValValLysAlaValCysValIleAsn ▪AspAla LysGlyThrValPhePhe Glu ▪GluSerSerGlyThrProValLysVal ▪ GlyGluValCysGlyLeu AlaLysGly ▪HisGlyPheHisValHisGluPhe Gly ▪AsnThrAsnGly CysMetSerSerGly ▪HisPheAsnProTyrGly LysGluHis ▪AlaPro ValAspGluAsnArgHisLeu ▪AspLeuGlyAsn IleGluAlaThrGly ▪ CysProThrLysValAsnIleThrAsp ▪LysIleThr LeuPheGlyAlaAsp Ser ▪IleGlyArgThrValValValHisAla ▪AlaAspAspLeuGlyGln GlyGlyHis ▪LeuSerLysSerThrGlyAsnAla Gly ▪ArgIleGlyCys GlyValIleGlyIle ▪LyS.