The investigations were carried out on m. longissimus of 24 lambs of both sexes representing the prolific Merinofmn breed (Mf) and F₁ crosses Texel x Mf (TMf) and Ille de France x Mf (IfMf). The lambs were fattened semi – intensively to reach 30-35 kg live weight. Samples of longissimus muscle were taken immediately after slaughter to categorize myofibers on STO, FTO and FTG types according to their metabolic and contractile properties. The percentage of muscle fiber types and their diameters, the muscle fibers number per area unit, as well as the intramuscular fatty tissue content were measured. As for these micro-structural parameters, no significant differences between genetic groups of lambs were stated. However, a following tendency was found: diameters of FTO and FTG muscle fiber types tended to increase, and muscle fiber numbers in the crosses with the share of meat breed had a tendency to decrease. Statistically significant differences were found in diameters of STO fibers related with sex. Female lambs showed greater diameters of their STO muscle fibers than the male lambs.