  Comparing the signals measured by the surface and underground scintillator
detectors of the Yakutsk Extensive Air Shower Array, we place upper limits on
the integral flux and the fraction of primary cosmic-ray photons with energies
E > 10^18 eV, E > 2*10^18 eV and E > 4*10^18 eV. The large collected statistics
of the showers measured by large-area muon detectors provides a sensitivity to
photon fractions < 10^-2, thus achieving precision previously unreachable at
ultra-high energies.
