Single‐crystal cobalt films from 50 to 350 Å thick have been grown on GaAs by molecular beam epitaxy techniques used previously to grow single‐crystal films of bcc Fe. Insitu reflection high‐energy electron diffraction during growth indicates that the crystallographic axes of the films are parallel to those of the (110)‐face GaAs substrates, and always shows single‐crystal diffraction patterns for films less than 50 Å thick. The surface net spacing is very close to what would be expected for a (110) face of bcc Co by extrapolating the lattice constants of Fe‐Co alloys to 100% Co. Vibrating sample magnetometer measurements in the plane show 〈001〉 axis easy, 〈111〉 intermediate, and 〈110〉 axis hard in accord with a negative K1. Ferromagnetic resonance (FMR) measurements at fixed frequency (9.5 and 34.6 GHz) and variable frequency (1–18 GHz) were performed on these films. The magnetic field H was in the film plane or perpendicular to the film for the fixed frequency FMR measurements. H was applied along the 〈110〉 axis for the variable frequency measurements. For the thickest single‐crystal film grown (357 Å) the following magnetic parameters were determined: 2K1/M=−1.08 kOe, g=2.16, 4πM=15.5 kG, and Hu =1.32 kOe. Hu is a strain‐induced uniaxial magnetic anisotropy field whose easy axis is along the 〈001〉 direction. The angular variation of the in‐plane FMR fields is consistent with cubic but not hexagonal Co.