A Candida albicans gene (CPH1) was cloned that encodes a protein homologous to Saccharomyces cerevisiae Ste12p, a transcription factor that is the target of the pheromone response mitogen-activated protein kinase cascade. CPH1 complements both the mating defect of ste12 haploids and the filamentous growth defect of ste12/ste12 diploids. Candida albicans strains without a functional CPH1 gene (cph1/cph1) show suppressed hyphal formation on solid medium. However, cph1/cph1 strains can still form hyphae in liquid culture and in response to serum. Thus, filamentous growth may be activated in C. albicans by the same signaling kinase cascade that activates Ste12p in S. cerevisiae; however, alternative pathways may exist in C. albicans.