  We discuss the applicability of the Wong formula for fusion cross sections in
a single-channel problem. To this end, we carry out a systematic study and
compare the approximate fusion cross sections with the exact results in a wide
mass region of reaction systems. We show that the deviation of the approximate
results from the exact cross sections is large for light systems, even though
the Wong formula provides a reasonable approximation for heavy systems. We also
discuss the energy dependence of the deviation, and show that for a given
projectile nucleus the critical energy, at which the deviation exceeds 5\% of
the exact cross sections, increases as a function of the mass number of the
target nucleus.
