In 14 of 21 infant hearts (66%) with aortic arch interruption between the left common carotid and left subclavian arteries (type B of Celoria and Patton), the right subclavian artery (SA) arose anomalously. In 9 (43%) it arose from the descending aorta and in 1 heart from the right pulmonary artery (5%). In 4 hearts (19%), it arose high in the neck from the right common carotid artery as a trifurcation with the internal and external carotid arteries, an anomaly not previously reported. In these latter cases, the SA descended down the neck to enter the right arm, with the right recurrent laryngeal nerve coursing around its take-off. Anomalous origin of the right SA is common in type B aortic arch interruption, and we believe this to be the result of a strong tendency for both fourth aortic arches to disappear early in development. If involution occurs in embryos of less than 14 mm crown-rump length, before the right ductus caroticus and dorsal segment of the right sixth arch have become attenuated, 3 alternate routes are available to the embryo to form a right SA.