An attempt is made to study the effect of a quantizing magnetic field on the effective electron mass in degenerate n-type narrow-gap semiconductors at low temperatures. It is found, taking n-Hg1-xCdxTe as an example, that the effective electron mass shows an oscillatory magnetic-field dependence as is expected because of the dependence of the effective mass in degenerate non-parabolic bands on Fermi energy which oscillates with changing magnetic field. The amplitude of oscillations is, however, found to be significantly influenced by the alloy composition whereas the period is found to be independent of the band non-parabolicity, i.e. of the compositional parameter in ternary semiconductors. © 1981 Springer-Verlag.