Aim: To present a case report of root canal therapy on a mandibular first molar with a supernumerary distolingual root (radix entomolaris). Summary: An additional distolingual root, the radix entomolaris is a major anatomical variant of the tworooted mandibular first molar. Awareness and understanding of this unusual root morphology contributes to the success of root canal therapy. This report discusses endodontic retreatment of a mandibular molar with a radix entomolaris, which is a rare macrostructure predominantly found in Caucasian populations.