A total of 152 chickens (45 d of age) were divided into 4 groups (38 chickens/group) on the basis of carcass weights: 1500-1550, 1600-1650, 1700-1750 and 1800-1850 g (Groups I-IV, respectively). Pectoral muscle on the left side of animals in groups I-IV were 3.1, 3.7, 4.9 and 4.8% heavier compared with right pectoral muscles, respectively (P < 0.05). Left suprachoracoid muscle in Groups I-IV were 3.8, 2.0, 3.1 and 3.3% heavier than suprachoracoid muscles on the right side, respectively. By increasing carcass weight, the weights of the left and right pectoral muscles increase significantly. When carcass weight was 1600-1650 g or heavier, the weights of left and right supracoracoid muscles either changed slightly or increased.