Hydatid cysts of the musculoskeletal system are rare. Unusual magnetic resonance imaging (MRI) findings of an infected primary hydatid cyst of the biceps femoris muscle are presented in a 40-year-old man on hemodialysis for chronic renal failure. No daughter cysts were present within the mother cyst cavity, but there was a fatty nodule which has not previously been described in a muscular hydatid cyst. Although the cyst was infected secondarily, no surrounding soft tissue inflammatory reaction was noted. Hydatid cysts should be included in the differential diagnosis of unusual soft-tissue masses in regions where the disease is endemic.