The mobility of ions in helium has been measured at 76.8 K and E/N values up to 15 Td, and at room temperature up to 300 Td. The experimental scatter is of the order of 0.1 - 0.2%. The experimental mobilities are compared with values calculated from a new and accurate ab initio potential (Røeggen et al 1996). The agreement between experimental and theoretical values is significantly better than in earlier studies of this ion - atom system, but some unexplained discrepancies of the order of 1% still exist.