Modulation‐dopedp‐AlGaAs/GaAs heterojunctions have been grown by molecular beam epitaxy. The effects of undoped AlGaAs spacer thickness on sheet carrier density and on Hall mobility have been investigated. A mobility of 97 000 cm2 V−1 s−1has been obtained at 4.2 K for a sheet density of 1.7×1011cm−2. This is the highest mobility reported for holes in III‐V compound semiconductors. A valence‐band offset of 210±30 meV was deduced for Al0.5Ga0.5As /GaAs heterojunctions.