Cubic crystal field Stark splittings have been calculated for the lowest term of the rare earth ions Ce 3+ through Sm 3+ and Tb 3+ through Yb 3+ for combined fourth and sixth order potentials. Energy level diagrams corresponding to eightfold coordination are given for seven choices of the ratio of sixth to fourth order terms, ranging from pure fourth order field to about five times the ratio to be expected on a point charge model. The splittings in different ions are compared on the assumption that the crystal field intensities and the expectation values for the fourth and sixth powers of the rare earth f-electron radius vary smoothly through the rare earth series. Experimental information on garnets suggests that the ratio of sixth to fourth order terms is about twice that to be expected on a point charge model for these crystals. For this ratio there is a marked tendency in ions to the right of gadolinium for the crystal field levels to cluster into two groups separated by a gap large compared with the spacings within each group. The total Stark splitting is found to be of comparable magnitude for all the rare earth ions, with somewhat larger values at the ends of the series than in the middle.