We report the observation of highly efficient laser emission from a single InAs layer with an effective thickness of 1.5 monolayers (ML) embedded in bulklike GaAs. Lasing action is obtained at the wavelength of the InAs thin-layer luminescence (870 nm) by cw optical pumping with a threshold power density of 0.9(3) kW/cm2 at 10 K. Gain measurements yield a very high material gain of 1.0(5)×104 cm-1 for the InAs layer when pumped with ~10 kW/cm2 at low temperatures. The 0 dimensional character of the emission as determined from cathodoluminescence and the absence of band-gap renormalization with increasing pump level speak for an excitonic mechanism of population inversion.