Molecular‐beam epitaxy was used to grow a 100 nm Al0.1Ga0.9N/100 nm GaN/500 nm Al0.1Ga0.9N double heterostructure on a 10‐&mgr;m‐thick GaN buffer layer grown with hydride vapor phase epitaxy on (0001) sapphire. Lasing from the 100 nm GaN active layer has been obtained at ∼359 nm at liquid‐nitrogen temperature (77 K) and at ∼365 nm at room temperature (295 K), using transverse optical pumping at 337.1 nm with a 600 ps transversely excited atmospheric pressure pulsed nitrogen laser. Threshold pump fluences were measured to be 0.3 and 0.5 mJ/cm2at 77 and 295 K, respectively, for a laser with 65 &mgr;m cavity length. In a laser of 23 &mgr;m cavity length, longitudinal cavity modes were observed with 0.56 nm spacing, corresponding to a group refractive index of 5.0 at the lasing wavelength. ©1996 American Institute of Physics.