Development is buffered against unpredictable environmental and genetic effects. Here, a molecular genetic analysis of one type of developmental homeostasis, the establishment of thoracic segmental identity under the control of the Ultrabithorax (Ubx) gene in Drosophila melanogaster, is presented. Flies were artificially selected for differential sensitivity to the induction of bithorax phenocopies by ether vapor, The experiments demonstrated that increased sensitivity to ether correlated with a loss of expression of UBX in the third thoracic imaginal discs and that a significant proportion of the genetic variation for transcriptional stability can be attributed to polymorphism in the Ubx gene.