Plasmids were isolated from erythromycin-resistant Corynebacterium diphtheriae and skin coryneforms. Six erythromycin-resistant C. diphtheriae strains, isolated from cutaneous lesions, all contained a 9.5-megadalton (Mdal) plasmid. Loss of resistance was associated with the deletion of a 1-Mdal segment from the plasmid or, less frequently, with loss of the plasmid. Two erythromycin-resistant diphtheroids were isolated from similar skin lesions. One contained a 38-Mdal plasmid that was lost in the conversion to erythromycin susceptibiliy. The other diphtheroid contained a 30- and a 14-Mdal plasmid. Erythromycin-susceptible derivatives of this strain were not recovered. Restriction enzyme analysis indicated that the 9.5-Mdal plasmids in the C. diphtheriae strains are very similar, if not identical, and that each of the deleted plasmids has lost the same 1-Mdal segment. However, the restriction patterns of the plasmids in the two diphtheroids are not closely related to each other nor to the plasmids in the C. diphtheriae strains.