Alfvén waves in bismuth have been studied in the frequency range 300 to 2000 Mc/sec, for propagation along the magnetic field. All the measurements were done on the same single crystal, and it was found that at the higher frequencies the real part of the dielectric constant is greater than the imaginary and that at lower frequencies the reverse is the case. This frequency dependence had a marked effect on the real part of the refractive index of the medium, which allowed one to measure, in addition to the usual charge-carrier mass density, some mean relaxation time of the charge carriers. The mass densities are in reasonable agreement with theory when the effective masses of the holes and electrons for bismuth from some previous experimental results are used, though there are some inconsistencies, which are discussed. A detailed analysis of the relaxation times proved impractical, but the magnitudes of the values agree well with other experimental results. The effect of Shubnikov-de Haas oscillations on the transmission was also studied. © 1966 The American Physical Society.