Many biological processes appear to be controlled by functionally redundant genes or pathways, but it has proven difficult to understand the nature of this redundancy. Here, we have analyzed a redundant regulatory interaction between the Drosophila transcription factor zeste and the homeotic gene Ultrabithorax. Mutations in zeste do not affect the cis-regulation of the endogenous Ultrabithorax gene; however, the expression of small Ultrabithorax promoter constructs is strongly dependent upon zeste. We show that this difference is due to redundant cis-regulatory elements in the Ultrabithorax gene, which presumably contain binding sites for factors that share the function of zeste. We also provide evidence suggesting that zeste and the gene encoding the GAGA factor have an overlapping function in regulating Ultrabithorax. Furthermore, we show that the zeste protein is bound at equal levels in vivo to a Ultrabithorax promoter construct, which zeste strongly activates, and to the identical promoter region in the endogenous Ultrabithorax gene, which zeste redundantly regulates. These results suggest that zeste is significantly active in the wild-type animal and not simply a factor that is induced as a back-up when other activators fail.