An electrodeless fluorescent lamp, suitable for substitution in place of existing incandescent lamps, comprises a gas filled toroidal envelope assembly removably mounted around a cylindrical ballast assembly. The ballast supplies radio frequency energy which induces electric current flow within the envelope and stimulates the emission of visible light. A toroidal transformer, which may have a ferrite core, is contained within a constriction on the envelope toroid and couples the energy to the gas. Heat generated within the envelope and transformer is conducted through the ballast to a radiator. The heated surfaces of the ballast and envelope form a chimney which induces air flow over the radiator.