Two genes known to control the determination of segmental identity in Drosophila melanogaster are polycomb and antennapedia. To identify additional genes involved in the determination of segmental identity, we have isolated dominant modifers (both suppressors and enhancers) of polycomb and/or antennapedia mutations. Sixty-four such modifier mutations have been recovered and mapped to 18 complementation groups. All of the mutations identify genes necessary for viability of the zygote. Six of the 18 genes that were identified by mutations that interact with polycomb and/or antennapedia have been previously characterized as homoeotic genes [i.e., Sex combs reduced (Scr), Brista (Ba), trithorax (trx), Polycomb (Pc), Polycomblike (Pcl), and Sex comb on midleg (Scm)]. Mutations in several of the additional loci identified here have also been shown to have homoeotic phenotypes.