Abstract. Forty 7-week-old ducks, 5 males and 5 females, from strains A44 and P66 and conservation flocks P33 and K2 were investigated. Their carcasses were dissected and chemical composition of the pectoralis major muscle was analysed. The carcasses of A44 and K2 drakes and ducks contained (%) significantly more pectoral muscles compared to the carcasses of P66 and P33 drakes and ducks. The carcasses of K2 males contained significantly less (%) thigh and lower thigh muscles than the carcasses of P66 and P33 males. The carcasses of K2 females had significantly less skin with subcutaneous fat compared to the carcasses of A44, P66 and P33 females. Pectoralis major muscle of drakes and ducks from conservation flocks contained more soluble collagen in total collagen than in animals from breeding strains. Moreover, the muscle of K2 males and females was found to contain more polyunsaturated fatty acids than in A44 males and P66 females, respectively. Pectoralis major muscle of drakes and ducks from both conservation flocks was characterized by a significantly lower energy value than in P66 drakes and ducks of both strains, respectively.