The Gli-like transcription factor TRA-1 of C. elegans promotes female development by repressing the transcription of male-specific genes. We have found that tra-1 interacts with tra-4, a previously uncharacterized gene that encodes a protein similar to the human proto-oncoprotein and transcriptional repressor PLZF. In this context, the TRA-4 protein functions with NASP-1, a C. elegans homolog of the mammalian histone chaperone NASP, and the histone deacetylase HDA-1. We also found that tra-4 is a member of the synMuv B group of genes, many of which encode homologs of components of the Drosophila Myb-Muv B transcriptional repressor complex, and that several synMuv B genes also promote female development. Based on these results, we propose that male-specific genes are repressed in C. elegans hermaphrodites by the combined action of TRA-1/Gli, a complex composed of TRA-4/PLZF-like, NASP, and HDA-1/HDAC, and synMuv B proteins. Similar interactions may function in sex determination and developmental regulation in other species.