Structural studies and ionic conductivity measurements have been carried out on a single crystal of beta-Pb0.9K0.1F1.9, which was synthesized using a dry method. The crystal is cubic : a==5.941(1) Angstrom, space group, Fm3m, z=4. The structure was refined to R=0.025 by least-square methods, using 83 unique observed reflections. The conductivity of this material is sigma T=1.4 (S cm(-1) K) at 300 K and the activation energy 0.24 eV. From these results it appears that the fluorine vacancies may be considered to be predominant mobile species in the extrinsic region. The phase is one of the best fluorine ion conductors.