Incipient carious lesions were examined by the SEM technique with the purpose of demonstrating the invasion of microorganisms under lesions without cavitation on the enamel surface. Microorganisms were seen to penetrate fairly deeply into the lesions. In some cases, they reached the dentino-enamel (DE) junction before any cavitation of the surface had ocurred. A gap between the enamel and the dentin could be completely filled with microorganisms. In some advanced attacks there was not only lateral spread of infection along the DE junction, but also outward (retrograde caries) as well as pulpward destruction, even before cavitation on the surface. Microorganisms seemed to be capable of removing enamel structures by direct action in a way similar to that of osteoclasts dissolving bone.