The relaxation oscillation frequency ofp‐substrate buried crescent InGaAsP lasers is measured as a function of the output power in the switched‐on state. The lasers are excited by a steplike current pulse at room temperature. The highest resonance frequency observed for a 350‐&mgr;m long cavity is 12.1 GHz equivalent to an intrinsic 3‐dB cutoff frequency of 18.8 GHz at an optical power of 9 mW/facet according to an injection current of 3.8 times threshold current, this is so far the best ever published value for ap‐substrate laser.