Distribution of the enzyme aldehyde oxidase in transformed haltere discs from the homoeotic bithorax series of mutants was investigated by histochemical means. The bithorax (bx) mutant, which transforms the anterior part of the haltere into an anterior wing blade, possesses in the haltere disc an aldehyde oxidase staining pattern similar that of the anterior side of the wing disc. The postbithorax (pbx) mutant, which transforms the posterior haltere into a structure resembling the posterior wing blade, reveals an aldehyde oxidase staining pattern in the haltere disc characteristic of the posterior side of the wing disc pouch. When both (bx3 pbx) mutants are present the haltere develops into a metathoracic wing. It is shown here that the transformed haltere disc closely resembles the previously established pattern in the wing disc with respect to aldehyde oxidase distribution. Change in the pattern of aldehyde oxidase in bithorax mutants signals alteration in gene expression which at least for this particular enzyme correlates well with the morphological transformation from haltere to wing. A possible correlation between pattern of enzyme activity and developmental compartmentalization has been discussed.