In Saccharomyces cerevisiae MATa cells, export of the a-factor mating pheromone is mediated by Ste6p, a member of the ATP-binding cassette (ABC) superfamily of transporters and a close homologue of mammalian multidrug transporter P-glycoproteins (Pgps). We have used functional complementation of a ste6delta mutation to isolate a gene encoding an ABC transporter capable of a-factor export from the pathogenic yeast, Candida albicans. This gene codes for a 1323-amino acid protein with an intramolecular duplicated structure, each repeated half containing six potential hydrophobic transmembrane segments and a hydrophilic domain with consensus sequences for an ATP-binding fold. The predicted protein displays significant sequence similarity to S. cerevisiae Ste6p and mammalian Pgps. The gene has been named HST6, for homologue of STE6. A high degree of structural conservation between the STE6 and the HST6 loci with respect to DNA sequence, physical linkage and transcriptional arrangement indicates that HST6 is the C. albicans orthologue of the S. cerevisiae STE6 gene. We show that the HST6 gene is transcribed in a haploid-specific manner in S. cerevisiae, consistent with the presence in its promoter of a consensus sequence for Mata1p-Matalpha2p binding known to mediate the repression of haploid-specific genes in S. cerevisiae diploid cells. In C. albicans, HST6 is expressed constitutively at high levels in the different cell types analysed (yeast, hyphae, white and opaque), demonstrating that HST6 transcription is not repressed in this diploid yeast, unlike in diploid S. cerevisiae, and suggesting a basic biological function for the Hst6p transporter in C. albicans. The strong similarity between Hst6p and the multidrug transporter Pgps also raises the possibility that Hst6p could be involved in resistance to antifungal drugs in C. albicans.