The d.c. electrical resistivity, rho, of pure LiNH4SO4 single crystals has been measured between 300 and 500 K in three successive runs. Anomalous behaviour before and at the transition point was observed and thermal annealing was necessary for reproducible results. The temperature dependence of rho along the polar axis yielded the values Delta E=0.54 and 1.48 eV and Delta E=1.95 eV for the energy activating the charge transport mechanisms in the ferroelectric and the paraelectric phases, respectively. A "pre-transition" phenomenon was observed while measuring rho along the a- and b-axes. The J-E characteristics indicated possible space-charge effects at low measuring fields. The mechanism of electrical conduction in the measuring range has been discussed.