The theory of generation of a heavy-hole exciton (2D exciton) from an electron-hole pair in a semiconductor quantum well (QW) is presented. Energy is conserved by emission of an acoustic phonon in the final state. The transition rate is calculated using a quasi 2D exciton-phonon interaction operator. In GaAs-AlxGa1-xAs quantum wells, the rate of generation is found to be 10 × 109 - 1 × 109 s-1 for quantum well widths 25 to 200 Å, which agrees well with recent experimental results.