Following carotid endarterectomy, a patient received chloramphenicol for a postsurgical complication. Subsequently his fingernails, which had been black, began to return to normal color. Since chloramphenicol-sensitive Proteus mirabilis was cultured from the nails, this organism may have been responsible for the black pigment. P mirabilis should be considered when a patient is seen with dark-brown or black nails.