We report the observation of Shubnikov–de Haas oscillations in superlattices of GaInAs and InP, showing evidence of two‐dimensional behavior. The electrong‐factor is deduced from both the criteria for resolution of a spin splitting by comparison with the broadening parameter &Ggr;, and from the tilted field method, and is shown to increase with increasing resolution of the Landau levels in a manner consistent with the theory of Ando and Uemura. In the ultraquantum limit, structure at &ngr;=1/2 and &ngr;=1/3 is observed.