Case Report: A male patient presented at a dental appointment complaining of spontaneuous pain in the left lower posterior region. Tooth 36 was diagnosed with irreversible pulpitis. Periapical tissue was found normal. A distolingual root was discovered through radiographic analysis. Taking under consideration such an anatomical deviation, prognosis is reserved. The root canal system was shaped with Protaper Gold files. Irrigation protocole included 5,25% sodium hypoclorite, 10% citric acid and 96% vol. ethyl alcohol, with sonic activation. Obturation was performed with guttapercha cones and resinous cement recurring to Tagger's hybrid technique. Discussion: Mandibular molar may presente additional roots located buccally (Radix Paramolaris) or lingually (Radix entomolaris) to the distal root. In order to clinically recognize a Radix Entomolaris it is required na rigorous evaluation of the tooth anatomy and pre-treatment radiographs with a diferente angle. In such cases a trapezoidal access cavity opening should be performed. Straight line access to the root canal system is required in order to achieve proper cleaning, shaping and obturation. Conclusion: Clinicians must be aware of anatomical deviations as Radix Entomolaris. This presentation describes how to adress such cases through proper clinical and radiographic diagnosis, adequate access cavity opening and shaping of the root canal system in order to promote the success of endodontic treatment. Keywords: Radix Entomolaris, mandibular molar, anatomical deviations, trapezoidal access cavity opening