The transport properties of molecular‐beam epitaxially grown InSb films doped with Er (atom densities NErfrom 4.3×1016cm−3to 3.6×1020cm−3) are reported at temperatures between 40 and 400 K. Er is a donor in InSb films grown under Sb to In flux ratios smaller than 1.1. At a flux ratio Sb/In=1.06, 25 atoms of Er give one extrinsic electron. The maximum extrinsic electron concentration achievable is ∼1.8×1017cm−3, obtained for NEr≳4×1018cm−3. Magnetoresistance measurements at 4.2 K show evidence for spin–disorder scattering of the electrons. The low‐temperature (T<150 K) electron mobility increases with doping concentration up to rare‐earth densities of 1×1018cm−3; at higher rare‐earth concentrations, the mobility decreases again. At room temperature, the mobility decreases monotonically with increasing Er concentration. We note the analogy between these results and the observations made on HgSe:Fe, a system in which the carrier mobilities might be enhanced by impurity charge ordering.