A three-month old baby presented to the Paediatric Department with a problem of abdominal distension. Clinically, he had a large right abdominal mass which on investigations suggested a teratomatous lesion. At operation, a foetus-like tumour mass was located in the retroperitoneal space. A diagnosis of foetus in foetu was made. A close differential of a retroperitoneal teratoma is discussed and comparison with confirmed cases made.