The transverse spontaneous emission is used to characterize the gain suppression in semiconductor lasers during their operation for the first time. The carrier heating and the spectral‐hole‐burning are distinguished by using their different spectral behavior. The measurements show a very strong carrier heating in strained multi‐quantum‐well lasers which do not appear in bulk devices. Moreover, the total nonlinear gain coefficient is measured by using a modulation method; the results show a high value of &egr;=16.10−17cm3for the strained multi quantum‐well laser and an usual value of &egr;=2.10−17cm3for the bulk, consistent with the measurement of gain suppression spectrum. ©1995 American Institute of Physics.