In 1995–1998, the frequency of selected virulences in the powdery mildew ( Blumeria graminis f. sp. tritici ) population on wheat in the Czech Republic was investigated. Samples of conidia were collected by a mobile version of a spore-trap. A total of 1739 one-colony isolates were analysed on a set of five varieties as differentials. High frequencies (> 50%)  were found for virulences Vm2, Vm3a, Vm5 and Vml7, lower frequencies(< 50%) for combinations Vm9,2,1 and Vm2,6 and for Vm4b. There were no significant changes in virulence frequencies except for genes Vm5 and Vm9,2,1 that showed one-year variations. Differences between the eastern part (Moravia) and western part (Bohemia) of the Czech Republic were mostly insignificant for most exam­ ined genes. A significantly higher virulence frequency was found in Moravia for genes Vm3a (in 1995) and Vml7 (in 1996), and in Bohemia for the combination Vm9,2,1 (in 1995). A significant correlation (tested by Kendall's tau B values) existed between Vm2 and Vm5 in 1997 and 1998, and between Vm4b and Vm5 in 1995 and 1998. Values of gametic disequilibrium were also studied. Changes in the complexity of isolate virulence were determined to be small and showed no definite trend.