I present evidence that natural selection biases synonymous codon usage to enhance the accuracy of protein synthesis in Drosophila melanogaster. Since the fitness cost of a translational misincorporation will depend on how the amino acid substitution affects protein function, selection for translational accuracy predicts an association between codon usage in DNA and functional constraint at the protein level. The frequency of preferred codons is significantly higher at codons conserved for amino acids than at nonconserved codons in 38 genes compared between D. melanogaster and Drosophila virilis or Drosophila pseudoobscura (Z = 5.93, P < 10(-6)). Preferred codon usage is also significantly higher in putative zinc-finger and homeodomain regions than in the rest of 28 D. melanogaster transcription factor encoding genes (Z = 8.38, P < 10(-6)). Mutational alternatives (within-gene differences in mutation rates, amino acid changes altering codon preference states, and doublet mutations at adjacent bases) do not appear to explain this association between synonymous codon usage and amino acid constraint.