ABSTRACTTop sirloin butts and strip loins were obtained from both sides of 20 U.S. Choice beef carcasses and cut into six steaks (2.54 cm thick). These steaks were assigned randomly to one of six aging treatments (0, 7, 14, 21, 28 or 35 days). Top sirloin steaks were less tender and more variable in tenderness than top loin steaks. This difference in tenderness appeared to be due to higher amounts of collagen in top sirloin steaks and a tendency toward lower collagen solubility, shorter sarcomere length and higher (less tender) fragmentation index.