This is a report of three cases in each of which an infant exhibited coarctation of the aorta at the level of a closed or closing ductus arteriosus. In all cases the collateral channels were poorly developed. It appears from the available evidence that death resulted from left ventricular congestive failure secondary to an obstructive hypertension. The development of collaterals to by-pass a coarctation of the aorta seems to depend upon the relation of the aortic coarctation to the aortic mouth of the ductus arteriosus. When the coarctation lies distal to the ductus, collaterals develop. When the coarctation lies proximal to the ductus, adequate collaterals do not develop before birth. When the coarctation lies opposite the aortic mouth of the ductus arteriosus, the development of collaterals during fetal life seems to depend upon the direction that the blood flowing in the ductus arteriosus takes. When the flow is into the aorta proximal to the coarctation, collaterals would be expected to develop; while when the ductal flow is into the descending aorta, adequate collaterals would not be expected to develop in the fetus. The three cases reported fall into the latter category.