The European rabbit flea Spilopsyllus cuniculi (Dale) bred successfully in wild rabbits on three properties in New South Wales and, within two breeding seasons, almost every rabbit shot within a quarter of a mile of a release site was infested.2. It was demonstrated that the flea transmitted myxoma virus in the field.3. In areas where more than 75% of the rabbits shot at the beginning of the breeding season were flea-infested and myxoma virus was present, populations failed to show the expected summer build-up.