A laser with an active volume consisting of only a single-quantum wire in the one-dimensional (1D) ground state is demonstrated. The single wire is formed quantum mechanically at the T-shaped intersections of a 14 nm Al0.07Ga0.93As quantum well and a 6 nm GaAs quantum well, and is embedded in a 1D single-mode optical waveguide. We observe single-mode lasing. from the quantum-wire ground state by optical pumping. The laser operates from 5 to 60 K, and has a low threshold pumping power of 5 mW at 5 K. (C) 2002 American Institute of physics.