A worldwide collection of strains of Cryphonectria parasitica was examined to draw a precise picture of the incidence and diversity of mitochondrial plasmids related to the plasmid pUG1. Amplification by specific PCR of 199 strains showed the presence of pUG1-like plasmids in 22% of the populations examined. The entire plasmid molecules were amplified by multiplex PCR and the products showed different RFLP patterns. The variability was mostly in a non-coding region of the molecule that has been sequenced in some representative strains, enabling the molecular evolution of the molecule to be elucidated. The data show that mitochondrial plasmids of C. parasitica comprise an almost homogeneous family (designated pCp) that can be divided into two clusters based on the presence/absence respectively of a 60 nucleotide region in North American and European plasmids.