Paracoccidioides brasiliensis, the etiologic agent of paracoccidioidomycosis, is a dimorphic fungus which is found as mycelia (M) at 26 degrees C and as yeasts (Y) at 37 degrees C, or after the invasion of host tissues. Although the dimorphic transition in P. brasiliensis and other dimorphic fungi is an essential step in the establishment of infection, the molecular events regulating this process are yet poorly understood. Since the differential gene expression is a well-known mechanism which plays a central role in the dimorphic transition as well as in other biological process, in this work we describe the identification and characterization of two differentially expressed P. brasiliensis hydrophobin cDNAs (Pbhyd1 and Pbhyd2). Hydrophobins are small hydrophobic proteins related to a variety of important functions in fungal biology, including cell growth, development, infection, and virulence. These two hydrophobin genes are present as single copy in P. brasiliensis genome and Northern blot analysis revealed that both mRNAs are mycelium-specific and highly accumulated during the first 24 h of M to Y transition.