Gain narrowing and lasing from a soluble, highly photoluminescent conjugated polymer,poly(2-butyl, 5-(2′-ethyl-hexyl)-1,4-phenylene vinylene)(BuEH-PPV), are compared using two resonant structures: planar waveguides and microcavities. The gain narrowing and lasing thresholds are comparable, 0.05–0.1 &mgr;J (10 ns pulse focused to∼1.5 mm). Gain narrowing is not observed in films on indium tin oxide (ITO) unless a cladding layer is placed between the BuEH-PPV and ITO. Single-mode microcavity lasers are obtained when a cavity resonance occurs at the wavelength where the gain of the polymer is maximum. ©1997 American Institute of Physics.