Spirochaetal organisms are found in skin specimens obtained by biopsy from a erythema chronicum migrans lesion. The histological picture shows a logical localization of a lymphohistiocytic cell infiltrate: deep dermal in the central papule and superficial in the erythematous border. The electron-microscopic characteristics of the micro-organisms in these specimens (regular waving appearance with coils every 0.8-1.2 microns, oblique striation of periplasmatic fibrils, cross-section of 0.3 micron, membranes) correspond to the spiral-like structure of Borreliae isolated from blood. The Borreliae seem to move freely through the matrix of the dermis and are not phagocytized. This brings on the typical clinical picture of the centrifugally spreading erythematous band.