Conjugated polymers have chemically tuneable opto-electronic properties and are easily processed, making them attractive materials for photonics applications(1,2). Conjugated polymer lasers, in a variety of resonator geometries such as microcavity(3), micro-ring(4), distributed feedback(5) and photonic bandgap(6) structures, have been fabricated using a range of coating and imprinting techniques. Currently, one-dimensional nanowires are emerging as promising candidates for integrated, subwavelength active and passive photonic devices(7-10). We report the first observation of optically pumped lasing in single conjugated polymer nanowires. The waveguide and resonator properties of each wire are characterized in the far optical field at room temperature. The end faces of the nanowire are optically flat and the nanowire acts as a cylindrical optical cavity, exhibiting axial Fabry-Perot mode structure in the emission spectrum. Above a threshold incident pump energy, the emission spectrum collapses to a single, sharp peak with an instrument-limited line width that is characteristic of single-mode excitonic laser action.