Twenty five-period Ga0.51In0.5P/(Al0.7Ga0.3)(0.5)ln(0.5)P quantum well (QW) structure was grown by MOVPE on GaAs substrates misoriented by 10degrees from (00 1) to (I I])A and fabricated into a microcavities with dielectric oxide mirrors. Lasing in the 625-650 nm spectral range with output power up to 9 W was achieved under scanning electron beam longitudinal pumping at room temperature. The laser wavelength, threshold and output power were found to depend critically on the alignment of QW period with both the cavity and the multi-quantum well (MQW) gain spectrum. The minimum threshold current density for a 40 keV electron energy was 8 A/cm(2). We have shown that low-threshold and high-power lasing requires the position of the QWs to coincide with the antinodes of the cavity resonance. Furthermore, the maximum of the gain spectrum, for ground state transitions, should also align with this etalon resonance. In order to control the lasing threshold to within 10% of its minimum, the MQW period should be tuned to the optimum value with an accuracy of about M (C) 2004 Elsevier B.V. All rights reserved.