The authors describe a case of accessory soleus muscle they have seen. It caused the development of a painful swelling on the medial side of the lower third of the leg with progressive equinovarus deformity of the foot. The upper attachment of the accessory muscle was in front of the normal muscle and distally was attached to the inner aspect of the calcaneus. The case was treated by transferring the accessory tendon to the tendo Achillis. 7 other cases reported in the literature are discussed in detail.