JeneNrreH, L. E. eNo MenrrN, A. H. rgj'|. The influence of sex, within breed-of-sire groupsr upon the histological properties of bovine longissimus dorsi muscle during postmortem aging. Can. J. Anim. Sci. 57: 7-i4.The longissimus dorsi (LD) muscles from 104 bovine carcasses (32 bulls, 36 heifers, and 36 steers) in three breed-of-sire groups (Charolais, Simmental and Chianina) were utilized to evaluate possible sex differences in histological properties ofthe LD muscle and the response of these properties to postmortem aging at 2 C. Significant (P<.05) sex differences in degree of LD fibeifragmentatio; w-ere observed within all groups. In two (simmental and chianina) of the three groups, the LD muscle fibers of steer carcasses fragmented to the greatest extent;ind in the other group (Charolais), the LD muscle fibers of bull cariasses fragmented to the lowest degree.Significant (P<.05) postrigor sex differences in LD sarcomere lengths and fiber diameters were observed within all groups. Sarcomere lengths were greatest for steers in all groups, and shortest for heifers in two (Charolaisind Simmental) of the three groups. Steers generally had smaller LD fiber diameters than bulls or heifers, but such sex differences were small. Following prolonged aging, heifers or steers had greater (P<.05) incidences of wavy LD fiberi than bults in two (Simmental and Charolais, respectively) of the three groups. While the histological response of LD muscle properties tended to be similar for all sexes both within and across groups, inherent differences in their histological muscle properties may require carcasseJof different sexes to be aged for different lengths of-time to achieve similar histological properties in the final product.On utilisa le muscle longissimus dorsi (LD) provenant de