We report on the intensity dependence of the responsivity in quantum well infrared photodetectors (QWIP). A strong reduction of the responsivity is observed already at small excitation powers for a QWIP withN=4periods. This nonlinearity is caused by a partial screening of the electric field across the main part of the active region. The intensity dependence is analyzed using a phenomenological approach, which allows us to calculate the nonlinearity from the measured dark current and responsivity. Applying this approach to a background-limited 50 period QWIP, we find that the nonlinearity is already present below1 mW cm−2,which is less than the thermal background. ©1997 American Institute of Physics.