Uniaxial compressive stress was applied to an AlGaAs/GaAs heterojunction. The uniaxial stress coefficients of sheet resistivity, sheet electron concentration, and mobility were obtained. The hydrostatic pressure coefficient of sheet resistivity was also obtained and was used to explain the different magnitude of the uniaxial stress coefficients of sheet electron concentration in the  and [11̄0] directions. We obtain a value for the piezoelectric constant e14 of AlAs to be -0.26 C/m2, compared to the value -0.225 C/m2 calculated by K. Hübner [Phys. Status Solidi B 57, 627 (1973)].