Single-mode edge emitting GaAs/A1GaAs quantum cascade microlasers at a wavelength of about 11.4 μm were realized by shortening the Fabry-P6rot cavity length. The spacing of the longitudinal resonator modes is inversely proportional to the cavity length. Stable single-mode emission with a side mode suppression ratio of about 19 dB at 85 K for a 150-μm-long device was demonstrated.