I REPORT here the correction of an error which, unfortunately, affects two articles of mine on extragalactic gamma radiation which appeared recently in Nature1,2. Equation (1) of ref. 1 should contain a factor of (1 + z)2 in the denominator instead of (1 + z)3. The same correction, applied to equation (12) of ref. 2, affects the index, α, of equation (15). This index then becomes 2 for a low density model and 1.5 for an Einstein–de Sitter model universe.