An increase by over three orders of magnitude from 0.58 m&OHgr; to 1.17 &OHgr; was observed at 4.0 K in the 3 GHz microwave surface resistance of ceramic YBa2Cu3O7−&dgr;following exposure to neutrons. The transport resistivity of an unirradiated pellet was linear in temperature down toTcwith a room‐temperature value of 2.13 m&OHgr; cm and a resistivity extrapolated to 4.0 K of 0.53 m&OHgr; cm. Following irradiation, the resistivity rose with decreasing temperature from a room‐temperature value of 1.5 &OHgr; cm to a maximum at around 45 K with little change evident atTc. These results, in conjunction with eddy current, susceptibility, iodometric titration, and thermally stimulated luminescence measurements on irradiated and unirradiated samples, collectively suggest that the effect of neutron irradiation has been to decrease intergranular coupling without the development of an insulating phase.