The effects of age and breed on growth parameters, carcass and muscle characteristics of bulls, slaughtered at 15, 19 and 24 months of age, were analysed in four French breeds: Aubrac (AU), Charolais (CH), Limousin (LI), and Salers (SA). Muscle characteristics were determined in three muscles : longissimus thoracis (U), semitendinosus (ST) and triceps brachii (TB). They included: (1) the % frequency, cross-sectional area and % area of fibre types, which were classified according to the contractile nature of the fibres and their metabolic properties (SO slow oxidative, FOG fast oxidative glycolytic and FG fast glycolytic); (2) the isocitrate dehydrogenase (ICDH) and lactate dehydrogenase (LDH) activities, representative of oxidative and glycolytic metabolism respectively; and (3) the total and insoluble collagen contents. In the fourbreeds, the average dailygain and the food efficiency decreased with age (P < 0.001). The carcass characteristics (muscle, fat and bone weights) increased with age (P < 0.001). The increase of muscle carcass weight with slaughter age was in parallel with the increase in cross-sectional area of individual muscle fibres. Oxidative fibre (SO and FOG) areas increased more between 15 and 24 months than glycolytic fibre (FG) area. Differences between muscles in increases in areas of muscle fibres were consistent: the increase was greater for TB than ST and LT The muscles studied became more slow and more oxidative above 19 months of age, as evidenced by the fact that the SO % frequency (P < 0.001) and % area (P < 0.001) and ICDH activity (P < 0.05) increased, and LDH activity decreased (P < 0.01). Insoluble collagen content decreased between 15 and 19 months (P < 0.001), and both total (P < 0.01) and insoluble (P < 0.001) collagen contents increased from 19 months. So carcass characteristics were modified between 15 and 24 months, and muscle characteristics were especially modified from 19 months of age. In addition, differences in slaughter data between breeds were clear and consistent, whereas those of muscle characteristics were few and not consistent.