The most usual form of launcher of an axial surface wave is a horn which acts as a mode transducer between a coaxial line and a surface waveguide.A full mathematical treatment of the horn presents considerable difficulty and therefore, as a step towards obtaining a solution for such a mode transducer, a launcher in the form of an annular slot in a conducting plate has been investigated. An exact mathematical solution has been obtained for the electromagnetic field at a large distance from the slot. Expressions for the surface-wave field and the radiated field were obtained and used to calculate the power carried by the two waves. The efficiency of launching of a surface wave was evaluated and the results were confirmed by experimental methods.