The stopping powers of helium ions transmitted through silicon in channelling directions (<110> and <111> axes and {111} plane) and in random directions for the energy range from about 0.1 MeV to 18 MeV have been determined. For the channelled cases, stopping cross sections have been calculated using both the position of the leading edge and the peak of the channelled energy spectra. Evidence is presented to support the view that the leading edge of the energy spectrum corresponds to the energy loss of the best channelled particles. It was found that the ratio of the channelled-to-random stopping, using either of the above-mentioned channelled stopping powers, is a function of the helium-ion energy havinga maximum value at the same energy at which the maximum in the stopping cross section is observed. For <110> channelling, this ratio for the best channelled ions varied from about 0.3 at 0.1 MeV to a maximum of about 0.55 and decreased to a limit of about 0.3 at high energies. The relation between the values of stopping powers determined in our transmission experiments and those applicable to depth calculations in Rutherford-scattering experiments is discussed.