The second harmonic of pulsed GaAs injection lasers was generated with the nonlinear crystal α-iodic acid (HIO3) phase matched by angular tuning. The injection laser was operated with an external cavity, and the HIO3crystal was placed inside the cavity to increase the conversion efficiency. The maximum harmonic power observed was 15 μW with a fundamental power of 4.5 watts inside the cavity. Since the lasing bandwidth was typically 30 Å and the calculated bandwidth for collinear harmonic generation was only 6.3 Å, a grating was used to narrow the lasing bandwidth to 3 Å. The expected large increase in harmonic power did not occur. It is shown that this is due to sum-frequency generation by laser modes lying outside the harmonic bandwidth. It is also found that the spectral width of the harmonic becomes bigger than the collinear bandwidth when a divergent laser beam is used. © 1970, IEEE. All rights reserved.