The energy-dependent equation for the slowing-down of energetic ions in fully ionized plasma is cast into a multigroup form. The resulting multigroup equation is a linear equation for the flux of the energetic ions and is easy to solve. It can take into account large-energy-transfer reactions preserving their discrete nature. The sensitivity of the multigroup approximation to the group width and weighting function used for generating group constants is investigated. It is found that few groups with simple, problem-independent weighting functions can yield results of reasonable accuracy. The relation between the multigroup and other methods for the solution of the slowing-down equation is discussed.