Magnetic measurements on Ga1-xMnxS in fields up to 25 T have been made at the National High Magnetic Field Laboratory using the cantilever technique. Ga1-xMnxS is in the new class of III–VI diluted magnetic semiconductors that are nominally two-dimensional (similar to mica). At temperatures below 30 K and in fields above 15 T the magnetization deviates from the previously reported linear behavior in lower field measurement. The diamagnetic constant for the host GaS has been measured to be M/H=-3.7×10-7 emu/g G. The magnetization of Ga1-xMnxS exhibits a small anisotropy with the easy axis along the c axis. Theoretical calculations for noninteracting moments were made and show reasonable agreement with the experimental data above 40 K. © 2002 American Institute of Physics.