the superficial muscle group of the posterior compartment of leg forms a powerful muscular mass in the calf. the two-headed gastrocnemius is the most superficial muscle of this group, and forms the proximal, most prominent part of the calf. gastrocnemius muscle exhibits numerous variations in the origin and/or insertion. the present report describes the incidental occurrence of the third/accessory head of gastrocnemius muscle in the left lower limb. during routine educational dissection of a 54-year old american female cadaver in the anatomy lab of the medical university of the americas, nevis, west indies, we found an unusual 10.8 cm long muscle in the left popliteal region, attached proximally to the lateral aspect of the popliteal surface of femur, and distally terminated as a tendon which was attached at the junction of the medial and lateral heads of gastrocnemius muscle. no such variation was found on the right side. this is described as a case of third / accessory head of gastrocnemius muscle.