It has been widely held that the steep temperature gradient in the lower thermosphere can support one or more surface modes of long-period gravity wave propagation, thereby acting as a ducting mechanism. This paper shows that such a ducting mechanism in fact does not operate in the earth's thermosphere. It is shown that the thermospheric gravity modes computed by Francis [1973> can instead be explained by internal waves undergoing total reflections in the lower thermosphere and weak partial reflections in the upper thermosphere due to viscosity and heat conduction effects. This ducting mechanism is, however, so weak that the usefulness of the concept of ducted thermospheric gravity modes appears to be limited. An alternative concept of freely propagating waves appears to have more practical applications.