The pigs’ skeletal muscles have a specific profile of muscle fibres, and this profile determines not only a functional character of each muscle, but, also, its technological and culinary properties. Five (5) experiments were performed during which samples of the muscles were taken upon the completed slaughter of fatteners, (not later than 20 minutes at the latest), and frozen in a liquid nitrogen. A histochemical reaction to determine the activity of NADH2 (diaphorase) was performed, and three (3) types of muscle fibres were distinguished: I – red fibres; IIA – intermediate fibres, and IIB – white fibres. It was stated that the number of muscle fibres was genetically determined, and changes occurring in this number could be caused by breed differences (cross breeding scheme). The fatteners cross-bred with Pietrain boars were characterized by the highest per cent content of white fibres of muscles, and by the lowest per cent content of red fibres compared with the fatteners cross-bred with Duroc boars. In the muscles of the latter fatteners, changes in the size of fibres occurred most frequently. The diameter of muscle fibres depended on the breed, feeding (or supplements, for example CLA isomers), body weight, and muscles type. The gender had no effect on the composition of individual types of muscle fibres, however, it impacted the diameter of white and intermediate fibres of muscles. Together with the increase in the body weight, there was an increase in the diameter of muscle fibres. The per cent content of muscle fibres, as well as their diameter depended not only on the muscle type, but, also, on the place within the muscle itself from which a sample was taken.