We have obtained the complete amino acid sequence of copper/zinc-containing superoxide dismutase (SOD, superoxide:superoxide oxidoreductase, EC 1.15.1.1) from Drosophila melanogaster. The sequence of this enzyme is also known for man, horse, cow, and the yeast Saccharomyces cerevisiae. The rate of evolution of this enzyme is far from constant. The number of amino acid substitutions per 100 residues per 100 million years is 30.9 when the three mammals are compared to each other, 10.6 when Drosophila is compared to the three mammals, and 5.8 when the yeast is compared to the four animals. The first value represents one of the fastest evolutionary rates for any protein, the second is similar to the globin rate, and the third is similar to some cytochromes and other slowly evolving proteins. Hence, SOD is not an acceptable evolutionary clock. Another peculiarity of this enzyme is that a two-amino-acid deletion must have occurred independently in the lineages going to the cow and to Drosophila. We conclude that using the primary structure of a single gene or protein to time evolutionary events or to reconstruct phylogenetic relationships is potentially fraught with error.