Two modifiers of bithorax1 phenotypic expression are described. An X-chromosome region is associated with sexual dimorphism in bx1 penetrance. It is hypothesized that sexual dimorphism is in part due to a lack of dosage compensation of the modifier, in males. A third chromosome region that segregates with the pink peach allele is implicated in mediating temperature sensitivity. By appropriate combinations of modifiers, both sexual dimorphism and temperature sensitivity can be greatly reduced.