To date, only very little is known about the molecular infection mechanisms of the head-blight pathogen of wheat, Fusarium graminearum (teleomorph Gibberella zeae). Here, we report on the isolation and characterization of the Fus3/Pmk1 mitogen-activated protein kinase homologue Gpmk1 from F. graminearum. Disruption of the gpmk1 gene in F. graminearum results in mutants that are reduced in conidial production, are sexually sterile and are fully apathogenic. This leads to the conclusion that gpmk1 is responsible for signal transduction processes taking place during the most important developmental stages in the life cycle of this fungal pathogen. Thus, Delta gpmk1 mutants are a useful tool to find other important genes involved in plant-infection mechanisms. Previously, only the trichothecene biosynthesis pathway was identified as a virulence factor in F. graminearum. Hence, Gpmk1 is now the second pathogenicity trait to be known in this important plant pathogen.