Enrichment of primary cosmic rays in heavy nuclei at energies higher than that corresponding to the knee point

Extensive atmospheric showers detected by the array of Moscow State Univers ity have been analyzed. Fluctuations of the number of muons and the gradien t of the lateral distribution of electrons have been investigated for showe rs of fixed size, and the mass composition of primary cosmic rays with ener gies in the range 10(15)-10(17) eV has been determined. Theoretical distrib utions have been computed on the basis of the updated version of the quark- gluon-string model. In agreement with our earlier results, primary cosmic r ays with energies above that which corresponds to the knee point show an in creased fraction of heavy nuclei.