We have observed the population of the second two‐dimensional electron subband in &dgr;‐doped Al0.48In0.52As/Ga0.47In0.53As heterostructures by Shubnikov–de Haas measurements. After illuminating the samples at low temperature, the electron density increases from 17.3 to 18.2×1011cm−2for the first subband and from 3.6 to 4.1×1011cm−2for the second subband. The population of the second subband begins when the first subband is filled at a density of 10.3×1011cm−2. The effective mass of the second subband is equal to (0.045±0.003)m0, indicating significant band nonparabolicity in the Ga0.47In0.53As well. ©1995 American Institute of Physics.