Symptomless invasion of black currant flowers by Botrytis cinerea was studied by fluorescence microscopy. Apparently healthy, newly abscissed flowers of many genotypes were found to contain infected ovules. This association was confirmed in inoculation experiments in which conidia germinated on stigmas and hyphae invaded styles, ovary walls and ovules and induced premature abscission. Substantially higher concentrations of ethylene were produced by flowers inoculated in the laboratory than in non-inoculated controls. The use of 2,5-norbornadiene confirmed that ethylene produced in response to infection was a major factor in premature flower abscission. The sensitivity of black currant genotypes to ethylene corresponded with their known susceptibility to 'running-off', with some Nordic germplasm being particularly non-responsive.