In order to isolate in planta-induced genes encoding putative pathogenicity factors of the late blight fungus Phytophthora infestans, a genomic library was differentially screened. For the differential hybridization, labeled first-strand cDNA synthesized on mRNA isolated from P. infestans-infected potato leaves and on mRNA isolated from the fungus grown in vitro were used as probes. This screening resulted in the isolation of the P. infestans calmodulin gene. The gene, designated calA, contains an open reading frame of 447 base pairs without introns and is unique in the P. infestans genome. The predicted amino acid sequence is 89.9-94.6% identical to calmodulins from higher eukaryotes, whereas the identity to calmodulins of higher fungi is significantly less (60.8-85.1%). Expression studies revealed that the P. infestans calA gene is constitutively expressed in in vitro grown mycelium. However, during pathogenesis on potato the level of P. infestans calmodulin mRNA is increased approximately fivefold.