Following the removal of the rodlet layer from aerial or submerged conidia of the entomopathogenic deuteromycetous fungus Beauveria bassiana, SDS-insoluble, formic-acid-extractable proteins were found in the residual cell wall material. Two major proteins (12.8 and 14.0 kDa) were extracted with formic acid from fractured aerial and submerged conidia but not from blastospores. Oxidation of the sample extracted by formic acid resulted in a single protein band (15.4 kDa) as judged by SDS-PAGE. Antibodies against this cell wall protein (cwp1) did not cross-react with cell wall extracts from the entomopathogenic deuteromycetous fungi Verticillium lecanii or Metarhizium anisopliae. Western blot analysis of two-dimensional gels revealed at least three acidic isoforms (pI 4.0-4.8) of cwp1. Immunohistological studies revealed that the cwp1 was primarily localized in the cell wall of aerial and submerged conidia but not in blastospores. Immunolocalization was possible only if the conidia were previously boiled in 5% (v/v) beta-mercaptoethanol. The N-terminal sequence of cwp1 showed no similarities with other published sequences. Our results suggest that at least two major species of SDS-insoluble, formic-acid-extractable proteins exist in cell walls of B. bassiana aerial or submerged conidia; one is the hydrophobin which occurs in the outermost rodlet layer and the other, cwp1, occurs primarily next to the rodlet layer.