In Switzerland, the incidence of European foulbrood (EFB), caused by the Gram-positive bacterium Melissococcus plutonius, has increased dramatically between 1997 and 2005 but the epidemiology, including the transmission of M. plutonius, remains poorly understood. In this study, the distribution of M. plutonius among bees originating from apiaries and colonies with and without symptoms of EFB was evaluated using a specific and sensitive hemi-nested PCR. In more than 90% of colonies without EFB symptoms located in apiaries with EFB symptoms, the bees were carriers of M. plutonius. In apiaries without EFB symptoms, but near apiaries with EFB symptoms, bees carrying M. plutonius were detected in about 30% of the colonies. In regions without European foulbrood history, all bee samples were negative. The proportion of adult bees carrying M. plutonius in colonies without symptoms appeared to increase when the distance to apiaries with clinical EFB symptoms decreased