The authors report the case of a 6-year-old girl with multi-organ hydatid disease, revealed by a complication: hydatid cyst of the right atrium. They emphasize the latency and clinical polymorphism of right cardiac sites of hydatid cyst and the severity of its prognosis. The particular site of this right atrial tumour and its relations with the septal leaflet of the tricuspid valve could be confused, on echocardiography, with myxoma of the tricuspid valve. The aetiology in this case was confirmed at surgery.