  A time-reversal symmetry relation is established for out-of-equilibrium
dilute or rarefied gases described by the fluctuating Boltzmann equation. The
relation is obtained from the associated coarse-grained master equation ruling
the random numbers of particles in cells of given position and velocity in the
single-particle phase space. The symmetry relation concerns the fluctuating
particle and energy currents of the gas flowing between reservoirs or
thermalizing surfaces at given particle densities or temperatures.
