We studied the genetic bases of threeHairy-wing (Hw 1,Hw Ua ,Hw 49c ) mutations mapping in the region of theachaete-scute complex (AS-C). Analysis of X-ray-induced revertants ofHw 1 andHw 49c uncoveredachaete andscute mutant phenotypes respectively. This indicates that theHw mutant phenotypes result from an excess of function of these genes of theachaete-scute complex (AS-C). The phenotypes of the differentHws show allelic specificity in the pattern of extrachaetes. In addition to these mutations, certain inversions and internal duplications of the AS-C also produce aHw-variegated phenotype, probably due to variegation or decompensation of the genes of the AS-C. The expressivity of the differentHws (mutation or variegation) is modulated by the number of doses of the AS-C present in the genome. A similar dose-dependent modulation is exerted by the transregulatory geneshairy andextramacrochaetae. We discuss these results on the basis of a regulation model of the expression of the AS-C.