In order to improve understanding of its diversity, 338 isolates of Cryphonectria parasitica, the causal agent of chestnut blight, were sampled from 10 chestnut populations throughout chestnut-growing coastal and continental areas of Croatia. Eighteen vegetative compatibility (VC) types were identified. The VC type EU-1 was the most widespread, comprising 42·9% of the isolates, followed by EU-2 (21%) and EU-12 (14·2%). In respect to the occurrence of the main VC types, the C. parasitica populations in Croatia combined features of both northwestern and southeastern European populations. Perithecia and mating-type ratios of approximately 1: 1 were found in all populations, suggesting that sexual reproduction of the fungus is common in Croatia. Natural hypovirulence was also evident in all populations, with incidence of hypovirus-infected isolates ranging from 12·7% in Istria-Buje to 66·6% in the continental part of the country. A total of 36 hypovirus-infected isolates sampled throughout Croatia were analysed in ORF-A and ORF-B by RT-PCR/RFLP analysis. All viral isolates belonged to the Italian subtype of Cryphonectria hypovirus 1 (CHV-1) and were closely related to the isolates found in other European countries. The RFLP patterns found were also identical or similar to the patterns of three isolates collected in Croatia 22 years ago, suggesting a slow evolution of the hypovirus.