Electrical resistivity and x‐ray studies were performed on a series of cerium samples, each of which contained in solution a 2‐at.% addition of one of the other rare earths (except promethium and ytterbium). The resistivity measurements showed that the amount of double hcp &bgr; phase formed at low temperatures varied in accord with the solute species present, the maximum amounts of &bgr; being associated with the solutes at the ends of the rare‐earth series and the minimum with those near the center of the series. The anomalously strong &bgr;‐forming tendency of the europium solute is explainable on the basis of atomic‐size and valence effects. The x‐ray studies revealed a varying tendency toward extrinsic stacking‐fault formation, which correlates with the trend in &bgr;‐phase formation. This correlation is attributed to the fact that both &bgr; cerium and extrinsic faults in fcc &ggr; cerium exhibit ABAC stacking of close‐packed planes. It is proposed that the observed trends in ABAC stacking stability are dependent on the number of unpaired 4felectrons of the solute atoms, with the stability being least when the number of these magnetic electrons is greatest.