An accessory muscle (flexor digitorum longus accessorius) was encountered in the deep posterior compartment of both legs of a 57-year-old male cadaver. The muscle originated with two heads from the medial margin of the tibia, lateral margin of the fibula, posterior intermuscular septum and the deep fascia at the distal part of the leg. Both heads came together just posterior and superficial to the tibial nerve, and converged into a slender tendon which traversed the tarsal tunnel in the vicinity of the neurovascular bundle to reach the sole of the foot. It terminated by merging into the tendon of the quadratus plantae muscle. The potential of such an anomalous muscle to lead to misinterpretations of the radiodiagnostic examinations and the fact that it can be one of the causes of tarsal tunnel syndrome should be borne in mind.