In order to determine the relative merits of the various rare earths as reactor control materials, a series of relative worth measurements was made in the Dresden Critical Assembly at Vallecitos Atomic Laboratory. Combinations of the oxides of five rare earths with highest thermal cross sections (dysprosium, erbium, europium, gadolinium, and samarium) were compared with cadmium and 2 per cent boron steel. Dy2O3, Gd2O3, and Sm2O3, separately and in combination, were found to be roughly equivalent in worth. Eu2O3 was found to be the strongest absorber. Er2O3 was found to be much less effective than the other materials studied.