Fluxes of Cygnus A relative to those of Cassiopeia A were determined at forty-seven frequencies between 1265 and 1500 Mc/s, in the hopes of detecting absorption by intergalactic neutral hydrogen. The r.m.s. deviation of individual flux ratios from a least-squares line is 0.82 per cent, including an apparent 50-Mc/s oscillation in the data, probably due to an unexplained instrumental effect. It may be concluded with some confidence that the opacity due to intergalactic hydrogen is less than 0.25 per cent, one-third of the upper limit found in an earlier experiment. If this value is accepted, it appears either that the intergalactic medium is hotter than 10 K (and presumably ionized) or that it is cooler (but ionized more than 42 per cent), or that its density is less than 10-29 gm . The latter value is inadequate by a factor of 2 to account for a deceleration parameter of unity.