Failures to control grey mould by dicarboximides on winter‐grown cucumbers in glasshouses in Israel were attributed to the development of resistance to these fungicides in populations of Botrytis cinerea. A survey of 18 glasshouses in the winter of 1983‐84 revealed that resistance is widespread where dicarboximides have been used extensively. During the summer, resistant B. cinerea populations often shift back to sensitivity; however, following few dicarboximide sprays in the subsequent winter, resistant strains reappear, increase rapidly and become dominant in the pathogen population. In two out of three glasshouse experiments, combination of chlorothalonil and dicarboximides in spray programmes slowed down the increase of resistant strains, whereas in a third experiment such a programme had no effect. The usefulness of chlorothalonil in fungicide‐alternation programme to suppress dicarboximide resistance is dicussed in relation to the fitness properties of established versus newly‐appearing resistant mutants.