The homeotic gene teashirt (tsh) is known to regulate segmental identity of the trunk region of the Drosophila embryo. Here we report a requirement for tsh function in the development of adult head structures. Animals homozygous for a viable tsh allele or heterozygous for various embryonic recessive lethal alleles displayed miniaturized maxillary palps, a phenotype characteristically induced by dominant gain-of-function mutations of Antennapedia (Antp) homeotic gene. Animals transheterozygous for tsh and Antp mutations displayed an enhanced antenna-to-leg and a striking reduced-eye phenotype suggesting aggravated ANTP misexpression in eye-antennal discs of these animals. In agreement with this, in the developing eye-antennal discs of the tsh mutant animals a significant amount of ANTP protein was detected overlapping the domains where tsh is normally expressed. These results suggest that tsh specifies adult head segments by repressing Antp expression.