A number of biochemical and genetic features shared by aldehyde oxidase and pyridoxal oxidase in Drosophila melanogaster indicate a close relationship between these enzymes. The present work shows that probable structural genes for these enzymes are within about 0.08 map unit of each other. Comparison with intensively studied regions of the genome suggests that this value is of the order of magnitude expected for adjacent functional units. Despite this close linkage, there is no indication of coordinate expression of the two enzymes. These results are consistent with the idea that each structural gene is under control of its own regulatory region, but alternative explanations are possible.