This work describes growth of the first thin ( 1 pm) epitaxial films of pure bismuth-antimony alloys using molecular beam epitaxy techniques. These structures were grown at elevated temperatures on single-crystal barium fluoride substrates of (111) orientation. Electron microscope observations show the films to be featureless and defect-free on the scale of 0.1 pm. The films grow with their trigonal axis parallel to the (11 1) axis of the substrate, and Laue-backscattering pictures show that they are epitaxial. Mobilities of alloys with x = 0 are of the order of 2 m 2 V " S" at room temperature and increase to over 10 at 20 K and 100 at liquid helium temperatures. These values are far superior to those of other bismuth films grown to date, and approach mobilities observed in single-crystal bismuth. The dependence of energy gap and c axis lattice constant on x is different from that in bulk alloys, which may be due to the effects of strain arising from the 3.6 % lattice mismatch between sample and substrate.