The structure of the Ophiostoma ulmi and Ophiostoma novo-ulmi population of Switzerland was analysed. 79 Swiss isolates were compared to reference strains from various countries. A dendrogram from RAPD profiles was produced. O. ulmi and O. novo-ulmi were clearly separated into two major clusters. O. novo-ulmi was further divided into the EAN and NAN races. This classification is consistent with subgroups by taxonomic parameters (growth rate, culture aspect, fertility barriers). The variation detected within the EAN and NAN races indicates a post-epidemic situation in Switzerland. A clonal population is present only in a restricted area in mid-eastern Switzerland, indicating a still active disease front.