Abstract In Cd 1− x Co x S and Cd 1− x Co x Se, each isolated Co 2+ ion behaves as an effective spin S= 3 2 . The axial anisotropy results in two Kramers doublets: the lower with S z = ±1 2 , and the higher with S z = ±3 2 . In a magnetic field H along the c-axis, a level crossing at a field H c leads to a change of the ground state from S z = -1 2 to S z = -3 2 . As a consequence, at the field H c a large magnetization step (MST) occurs if the temperature is sufficiently low. This MST, arising from isolated Co 2+ ions in an axial anisotropy, was observed near 30 mK. A new force magnetometer operating in a plastic dilution refrigerator was used.