The number of muscle fiber nuclei per fiber in semitendinosus muscle showed increasing tendency during growth. The only significant increase was from 6 to 8 months of age. The number of nuclei was different between the two metabolic types of cattle only at 6 months of age, whereas the total mean in CH bulls was higher than in H bulls due to higher values in the fiber type IIB (Table 4). A small difference in type IIA fiber type was seen at 13 months of age, again with a higher number of nuclei in CH bulls.