Pneumocystis carinii (Pc) adhesion to alveolar epithelial cells is well established and is thought to be a prerequisite for the initiation of Pneumocystis pneumonia. Pc binding events occur in part through the major Pc surface glycoprotein Msg, as well as an integrin-like molecule termed PcInt1. Recent data from the Pc sequencing project also demonstrate DNA sequences homologous to other genes important in Candida spp. binding to mammalian host cells, as well as organism binding to polystyrene surfaces and in biofilm formation. One of these genes, flo8, a transcription factor needed for downstream cAMP/PKA-pathway-mediated activation of the major adhesion/flocculin Flo11 in yeast, was cloned from a Pc cDNA library utilizing a partial sequence available in the Pc genome database. A CHEF blot of Pc genomic DNA yielded a single band providing evidence this gene is present in the organism. BLASTP analysis of the predicted protein demonstrated 41 % homology to the Saccharomyces cerevisiae Flo8. Northern blotting demonstrated greatest expression at pH 6.0-8.0, pH comparable to reported fungal biofilm milieu. Western blot and immunoprecipitation assays of PcFlo8 protein in isolated cyst and tropic life forms confirmed the presence of the cognate protein in these Pc life forms. Heterologous expression of Pcflo8 cDNA in flo8 Delta-deficient yeast strains demonstrated that the Pcflo8 was able to restore yeast binding to polystyrene and invasive growth of yeast flo8 Delta cells. Furthermore, Pcflo8 promoted yeast binding to HEK293 human epithelial cells, strengthening its functional classification as a Flo8 transcription factor. Taken together, these data suggest that PcFlo8 is expressed by Pc and may exert activity in organism adhesion and biofilm formation.