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We present the interpretation of the muon and scintillation signals ofultra-high-energy air showers observed by AGASA and Yakutsk extensive airshower array experiments. We consider case-by-case ten highest energy eventswith known muon content and conclude that at the 95% confidence level (C.L.)none of them was induced by a primary photon. Taking into account statisticalfluctuations and differences in the energy estimation of proton and photonprimaries, we derive an upper limit of 36% at 95% C.L. on the fraction ofprimary photons in the cosmic-ray flux above 10^20 eV. This result disfavorsthe Z-burst and superheavy dark-matter solutions to the GZK-cutoff problem.Comment: revtex, 8 pages, 4 figure

Upper limit on the ultrahigh-energy photon flux from AGASA and Yakutsk data