  We report anisotropic resistivity measurements on a
La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ single crystal over a temperature $T$ range
from 2 to 400 K and in magnetic fields $H$ up to 14 T. For $T\geq 218$ K, the
temperature dependence of the zero-field in-plane $\rho_{ab}(T)$ resistivity
obeys the adiabatic small polaron hopping mechanism, while the out-of-plane
$\rho_{c}(T)$ resistivity can be ascribed by an Arrhenius law with the same
activation energy. Considering the magnetic character of the polarons and the
close correlation between the resistivity and magnetization, we developed a
model which allows the determination of $\rho_{ab,c}(H,T)$. The excellent
agreement of the calculations with the measurements indicates that small
polarons play an essential role in the electrical transport properties in the
paramagnetic phase of bilayer manganites.
