The requirement for the trithorax gene during imaginal cell proliferation has been analysed by clonal analysis. Clones carrying the allelic Regulator of bithorax and induced by mitotic recombination at different developmental stages. The results indicate that the trithorax gene is required at least until the beginning of the third larval instar to ensure the correct differentiation of head, thoracic, and abdominal structures, and are consistent with a role for the gene in maintaining the appropriate levels of Antennapedia and bithorax complex expression.