Homeotic genes determine the identities of metameres in Drosophila. We have examined functional aspects of the homeotic gene teashirt by ectopically expressing its product under the control of a heat-shock promoter during embryogenesis. Our results confirm that the gene is critical for segmental identity of the larva. Under mild heat-shock conditions, the Teashirt protein induces an almost complete transformation of the labial to prothoracic segmental identity, when expressed before 8 hours of development. Positive autoregulation of the endogenous teashirt gene and the presence of Sex combs reduced protein in the labium explain this homeosis. Patterns in the maxillary and a more anterior head segment are partly replaced with trunk ones. Additional Teashirt protein has no effect on the identity of the trunk segments where the gene is normally expressed; teashirt function is overridden by some homeotic complex acting in the posterior trunk. Strong heat-shock regimes provoke novel defects: ectopic sense organs differentiate in posterior abdominal segments and trunk pattern elements differentiate in the ninth abdominal segment. Teashirt acts in a partially redundant way with certain homeotic complex proteins but co-operates with them for the establishment of specific segment types. We suggest that Teashirt and HOM-C proteins regulate common sets of downstream target genes.