Hydatid disease is rare parasitic illness with commonest site of occurrence being lung and liver. Rarely it is reported to be infecting in brain parenchyma, but intra–thoracic extra pulmonary origin of mediastinal hydatid disease is very rare and difficult to speculate. We reporter a rare case of mediastinalhydatid disease in a 53 year old male resenting with cough breathlessness and intermittent fever. Only after the thoracotomy and careful observation origin of the cyst was traced to thymus. Large well defined multilocular cystic lesion of 27X17X10 cm was resected and found to be hydatid cyst. The post operative progress of patient was uneventful.