Penicillium marneffei is an emerging fungal pathogen of humans, in particular those who are immunocompromised. P. marneffei has the capacity to alternate between a hyphal and a yeast growth form, a process known as dimorphic switching, in response to temperature. P. marneffei grows in the hyphal form at 25/C and in the yeast form at 37/C. The hyphal form produces conidia which are likely to be the infectious agent while the yeast growth form is the pathogenic form found in infected patients. These yeast cells exist intracellularly in the mononuclear phagocyte system of the host. The molecular events which establish and maintain the developmental states and control of the dimorphic switching process in P. marneffei are poorly understood. The abaA gene is a member of the ATTS class of transcriptional regulators which control developmental processes in eukaryotes. In P. marneffei and Aspergillus nidulans, abaA is a key transcriptional regulator of asexual development (conidiation) and in particular phialide differentiation. In addition, P. marneffei abaA controls yeast cell morphogenesis during the dimorphic switch, and mutants produce aberrant multinucleate yeast cells. In Saccharomyces cerevisiae and Candida albicans, the abaA homologue TEC1 regulates filamentation during pseudohyphal growth and hyphal morphogenesis, respectively. The regulatory pathway in which abaA operates is very well characterised for conidiation in A. nidulans. An extensive analysis of the upstream and downstream factors of this pathway has been conducted to understand which elements of this pathway are shared during conidiation and dimorphic switching. In addition, an examination of the regulatory signals that control abaA expression during conidiation and dimorphic switching has been performed. The data show that the brlA gene encodes a C2H2 zinc finger transcriptional regulator which is known to regulate abaA expression does not control dimorphic switching. Instead, promoter analysis has defined a newly evolved region which regulates abaA expression during yeast cell morphogenesis.