We report the anatomic variations found in four additional cases of trisomy 13. Data from these and four previous cases [Colacino and Pettersen, 1978] are utilized to define a muscle phenotype. Previously unreported defects include the bilateral presence of cervical ribs and the bilateral absence of 12th thoracic ribs in five of the eight cases. One unusual developmental defect of the great vessels is also described. The findings suggest that a definitive diagnosis of trisomy 13 can be made on the basis of six muscle variations.