The codon bias of two Aedes mosquito species was examined using a sign test. In general, there appeared to be some preference for C + G at the third base position, although this was not statistically significant. While amino acids such as phenylalanine and tyrosine clearly displayed biases, others such as valine and serine appeared to have little or no bias for any particular codon. Three homologous genes of Aedes aegypti and Drosophila melanogaster were compared using the chi-square test and the codon bias of the two species compared. Drosophila melanogaster was found to have a much stronger bias for C + G at the third base position compared to Aedes. The implications and usefulness of the codon bias are discussed.