By means of transmission electron microscopy (TEM), the percentage of myosatellite cells was shown to decrease from about 6% in carp of 5 cm standard length (SL) to less than 1% in carp larger than 18 cm SL. The ratio between muscle nuclei and non muscle nuclei remained constant. These TEM data, combined with data on the amount of DNA per gram of muscle tissue and per nucleus, were used to calculate the numbers of myosatellite cells per gram of tissue (TEM-DNA method). Total numbers of myosatellite cells could be calculated from the TEM-DNA data and the calculated amounts of muscle tissue per fish. After a slight initial increase, the total number of myosatellite cells in the white axial muscle of a carp appears to be rather constant during the growth phase of the fish. But the myosatellite cells become more and more diluted over an increasing number of myonuclei with age. In addition, the reliability of two new light microscopic methods for the determination of numbers of myosatellite cells was examined. The percentages of myosatellite cells were determined by counting the numbers of total nuclei and of heterochromatic nuclei situated inside the muscle fibers' basal laminae which were stained using an antibody against laminin. These percentages were not significantly different from those determined with TEM. The yield of myosatellite cells per gram of muscle tissue, isolated with a previously developed dissociation method, showed a direct relation to the number of myosatellite cells calculated to be present in the tissue (TEM-DNA method), but at a 1% level. Both methods are alternative ways to determine numbers of myosatellite cells when it is impossible or difficult to use TEM (e.g., large sample sizes, combination with immunohistochemical methods).