Exposure of a total of more than 5 000 susceptible plants of barley, wheat and oats at Blåvandshuk, the most westerly point of Jutland, Denmark, for periods of several hours when air mass movements from the British Isles across the North Sea occurred, resulted in infections ofErysiphe graminisf.sp.hordei, E. graminisf.sp.tritici, E. graminisf.sp.avenae, Puccinia reconditaf.sp.triticiandP. hordei.Single colony isolates ofErysiphe graminisf.sp.hordeifrom exposures at different sites in 1976 were tested on the barley variety Sultan. The results strongly indicate that the frequency of the gene for virulence on Sultan in the mildew populations in North Jutland is influenced by the high frequency of the same gene for virulence in the mildew populations borne by wind from the British Isles. Zero or low numbers of infections on exposed plants at Blåvandshuk were generally associated with rainy weather at the source area. It is concluded that ideal conditions for both spore release in the British Isles and wind transport across the North Sea occur rather rarely.