Three statistics (%GC, GC-skew, and AT-skew) can be used to describe the overall patterns of nucleotide composition in DNA sequences. Fourfold degenerate third codon positions from 16 animal mitochondrial genomes were analyzed. The overall composition, as measured by %GC, varies from 3.6 %GC in the honeybee to 47.2 %GC in human mtDNA. Compositional differences between strands of the mitochondrial genome were quantified using the two skew statistics presented in this paper. Strand-specific distribution of bases varies among animal taxa independently of overall %GC. Compositional patterns reflect the substitution process. Description of these patterns may aid in the formation of hypotheses about substitutional mechanisms.