The isolates of Cylindrocarpon destructans used in the present study were selected from a collection of isolates from rootstock nurseries and grapevines showing symptoms of decline in Portugal during the last few years. These isolates were compared on the basis of gross cultural morphology, micromorphology and pathogenicity. The most variable parameter was colony colour. Analysis of micro- and macroconidial size of cultures on “Spezieller Nährstoffarmer Agar” plus 0.1% yeast extract medium (SNAY) revealed that all isolates except one, could be clustered in a single group. On carnation leaf agar (CLA) medium, however, all isolates clustered in a single group. All C. destructans isolates were pathogenic on rooted grapevine cuttings of cv. Periquita under greenhouse conditions. Internal black-foot symptoms were seen in inoculated plants and the pathogen was reisolated from 60 to 100% of these plants. All C. destructans isolates caused a significant (P=0.05) decrease in the height of inoculated plants, and the majority also caused a significant (P=0.05) reduction in the number of roots.