The aim of the present study was to investigate observed and predicted changes in virulence gene frequencies in a local aerial powdery mildew population subject to selection by different host cultivars in a local barley area. Observed changes were based on genotypic frequencies obtained through a survey comprising 11 virulence loc. Predictions were based on a model where selection forces were estimated through detailed mapping in the local area of host cultivars and their resistance genes, and taking into account the changes in distribution of host cultivars during the year caused by growth of both autumn- and spring-sown host crops. Large differences in gene frequencies were observed between spore samples collected in periods with a different distribution of host cultivars, whereas only minor differences were observed between spore samples collected at different times within periods with a constant distribution of host cultivars. Significant changes in gene frequencies were observed for virulence genes subject to strong direct selection as well as for genes subject mainly to indirect selection (hitchhiking). These patterns of changes were generally as predicted from the model. The influence of local selection forces on the local aerial population was demonstrated by a sample of aerial spores with a unique genotypic distribution collected in summer 1989. The results emphasize the importance of knowledge concerning sources of samples when analyzing the dynamics in pathogen populations. Further, the results imply that virulence survey data, which are based on random spore samples collected in regions with a uniform distribution of the different host cultivars, form the most favourable basis for predicting the changes in the genetic composition of aerial powdery mildew populations.