The use of dielectric waveguides in the form of small fibers as the mode selector in optical masers is considered. The fibers consist of a core of index of refraction n1 which contains the maser material, surrounded by a cladding of lower index n2. A comparison is made with the Fabry‐Perot interferometer used as a cavity. The principal advantages of the fiber for maser applications are the mode selection and the stronger mode coupling. It is shown that for core diameters just small enough to support only the two HE11 modes, the fraction of spontaneous emissions into the waveguide modes is given approximately by 1.4 (n1-n2)/(n1+n2). This could make maser action possible at much lower power levels. The major disadvantage is the difficulty of pumping into the small volume of the fiber. Schemes to overcome this difficulty are discussed.