Host-parasite interactions of Agaricus bisporus and Verticillium fungicola were studied in paired-cultures. Different growth reactions were found according to the culture medium used, but no apparent alterations could be detected by light and electron microscopy in A. bisporus vegetative mycelia. In vitro, different hydrolytic enzymes are produced by V. fungicola in cultures with A. bisporus vegetative mycelial cell walls as the only carbon source. In vitro, this enzyme preparation is able to partially digest isolated A. bisporus vegetative mycelial cell walls. These results suggest that although apparent attachment of the mycoparasite to the host cell surface seems to occur, there is no in vivo A. bisporus vegetative mycelial cell wall degradation.