Optical bistability in a single quantum well waveguide resonator in the InP/InGaAsP material system is demonstrated. High contrast ratio (≳3:1) and switching at low power (≊1.5 mW) was obtained by using a single quantum well at a wavelength near the maximum nonlinear refractive index change and by depositing a reflective coating on the rear end of the waveguide. Clear optical bistability is observed in a wide wavelength range of 1480–1525 nm. Due to the low input power, cw operation of the device was possible at room temperature. ©1995 American Institute of Physics.