The diversity of Eutypa lata was studied in 56 isolates collected from Languedoc-Roussillon, France. The pathogenicity of isolates varied within a very broad range and was not influenced by the cultivar from which they originated. Using 27 RAPD markers, each isolate was identified as an unique genotype. Gene diversity averaged 0·292 for the regional population. Lack of gametic disequilibrium between RAPD loci supported the hypothesis of random mating. The RAPD analysis did not allow subdivision of the population according to geographic location, vine cv. or pathogenicity of E. lata. The composition of the community of fungi was studied in 116 vines showing eutypa dieback symptoms collected at several localities in the region. E. lata was recovered from 80% of the samples, from the sectorial brown lesion characteristic of the disease. This lesion was associated in half of the vines with a central zone of discoloured hard wood and/or a light-coloured lesion of soft consistency. E. lata was isolated alone from 124% of the vines and was associated in 67% of the vines with one, two or three of the following fungi: Sphaeropsis malorum, Phomopsis viticola, Gliocladium roseum, Phaeoacremonium chlamydosporum, P. aleophilum and Phellinus igniarius. The variation in symptoms observed in the vineyard may be partly explained by the wide genetic diversity in E. lata and differences in the composition and functioning of the fungal community.