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Diffuse ultrahigh energy γ-radiation can arise from a variety of astrophysical sources, including the interaction of 10 20 eV cosmic rays with the 3 K microwave background radiation or the collapse of topological defects created in the early Universe. We describe a sensitive search for diffuse γrays at ultrahigh energies using the CASA-MIA experiment. An isotropic flux of radiation is not detected, and we place stringent upper limits on the fraction of the γ-ray component relative to cosmic rays (< 10 −4 ) at energies from 5.7 × 10 14 eV to 5.5 × 10 16 eV. This result represents the first comprehensive constraint on the γ-ray flux at these energies.

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