A Na2dimer laser which is excited longitudinally by a single‐frequency Ar+‐ion laser at 488.1 nm exhibits a very narrow gain profile in forward direction of the pump beam. Using linear resonators with lengths ≲2 m, the gain profile can be appreciably narrower than the spacing of adjacent axial resonator modes. Only if a resonator mode coincides with the gain profile, laser oscillation is shown to occur. This leads to a very unstable emission with low efficiency. Possibilities to achieve controlled emission are demonstrated.