Tunable narrow‐band Lyman‐alpha radiation (∼1216 A˚) has been generated by frequency tripling in krypton. A near‐diffraction‐limited tunable 15‐nsec 10‐MW input at 3648 A˚ was used to generate ∼3.6×1011photons (60 W) per pulse at 1216 A˚. Measurements of the output as a function of gas pressure give the refractive index for krypton at 1216 A˚, demonstrating its negatively dispersive character.