We report the operation of a photopumped blue Znx′Cdy′Mg1-x′-y′Se/ZnxCdyMg1-x-ySe separate confinement heterostructure single quantum well laser grown lattice matched to InP with a relatively thick quaternary quantum well (∼50 Å). Laser emission at 492 nm in the blue was observed. The lasing linewidth is about 5 nm. Based on the temperature dependency of the threshold pumping intensity, the characteristic temperature (T0) was determined. We also studied a photopumped laser with a similar structure, where the only difference was the quaternary ZnxCdyMg1-x-ySe quantum well composition, having laser emission in the green. Comparison of the threshold pumping intensity and T0 for the blue and green lasers shows a lower threshold pumping intensity and higher T0 for the green laser. We explain these results on the basis of the difference in carrier confinement between these two structures. An Arrhenius treatment of the temperature dependency of the blue laser threshold pumping intensity gives an activation energy Ea very close to the band gap energy difference between the cladding layer and the quantum well in the conduction band. This points to a carrier loss process through thermalization into the cladding layer. © 2004 American Institute of Physics.