A DnaJ-like gene, Cpdj1, a molecular chaperone and regulator of Hsp70 in Cryphonectria parasitica, was characterized. The protein product of Cpdj1 gene consists of 379 amino acids with a predicted molecular mass of 40.6 kDa and a pI of 7.79. The deduced protein sequence revealed preservation of the conserved hall-mark J-region and exhibited high homology to all known DnaJ-like proteins. Disruption of the Cpdj1 gene resulted in slow growth and produced colonies characterized by retarded growth and a deep orange color. Accordingly, reduced virulence of the Cpdj1- mutant was observed. This reduced growth rate was magnified when the Cpdj1- mutant was cultured under heat-stress conditions. Reduced conidiation was also observed in the Cpdj1- mutant, indicating that Cpdj1 gene, although not essential for cell viability, is required for appropriate cellular processes including growth and sporulation. Northern analysis showed that Cpdj1 was constitutively expressed, and when the culture was subject to high temperature, a strong induction of the transcript was observed. No significant difference in the expression and induction pattern of Cpdj1 was observed between virus-free EP155/2 and virus-infected hypovirulent UEP1 strains. However, further severe defects in mycelia growth and conidiation were observed in the hypovirus-infected Cpdj1- mutant suggesting that the presence of Cpdj1 is required for mycelia growth and sporulation of the hypovirus-infected strain.