A tentative classification of melanocytic tumors of dogs is proposed. A canine melanocytic nevus with microscopic features similar to an active human junction nevus undergoing malignant transformation is described. Canine compound nevi appear to originate from melanocytes that have dropped off the epidermis or hair follicles; malignant transformation may occur. The life cycle of melanocytic nevi of dogs is unknown. Fibrous dermal melanocytoma of the dog appears to be equivalent to the "common" blue nevus of man. In dogs, nearly all oral melanocytic tumors are malignant, while most cutaneous lesions are benign. The ammoniacal silver nitrate technique is useful in demonstrating premelanin in many amelanotic melanomas of dogs. Canine malignant melanoma may originate de novo from melanocytes in epidermis and oral epithelium, and from junctional or dermal elements of canine melanocytic nevi. Differences and similarities between canine and human malignant melanomas are cited.