We have investigated the characteristics of green–blue II–VI semiconductor diode lasers at room temperature in high magnetic fields. Small spectral shifts, less than 2 meV during the sweep of a field up to 29 T, were measured in ZnCdSe/ZnSSe single quantum well index guided diode devices. This behavior is in contrast with expectations for a noninteracting electron-hole gas in a quantum well system at comparable pair densities(∼1012 cm−2),implying that the optical gain spectrum shifts diamagnetically. The measured diamagnetic shifts are in close agreement with the low injection electroluminescence and absorption spectra at then=1heavy-hole excitonic transition of the ZnCdSe QW. ©1997 American Institute of Physics.