Continous wave operation of interband cascade lasers based on type-II InAs/Ga(In)Sb heterostructures is reported. The interband cascade lasers, consisting of 23 cascaded active regions, lased at wavelengths near 4 mu m. continuous wave operation was observed from 60-110 mu m wide by 1.02 mm long mesa-stripe lasers at temperatures up to 70 K.