  The electrical resistivity $\rho$ and upper critical field $H_{c2}$ were
measured in MgB$_2$ disordered by nuclear reactor neutrons. We found that
superconducting temperature $T_c$ decreases under irradiation from 40 to 5 K.
Despite strong disordering (more than 10 dpa (displacements per atom)), the
initial crystal structure is preserved. The residual resistivity $\rho_0$
increases from 0.35 to 2 m$\Omega$ cm while $-dH_{c2}/dT$ remains approximately
unchanged ($-dH_{c2}/dT \approx 0.5$ T/K), and such behavior may be interpreted
as decrease in the density of electronic states.
