The C-shaped configuration of the root and root canal system is an unusual anatomical variation in mandibular first and second premolars. It is categorised as a 'C' due to the transverse morphology of the root and the root canal. The main anatomical characteristic is the presence of an invagination groove caused by Hertwig epithelial sheath faults during root development. This case report describes the root canal treatment of a C-shaped configuration of a mandibular first premolar, identified by means of cone beam computed tomography. Schlagwörter: anatomical variation, C-shaped root, C-shaped root canal, cone beam computed tomography, dental anomalies, mandibular premolar, tomography, tooth morphology