The three independent magnetoelectric susceptibilities of GdAlO3 have been measured in the critical region 6×10-5<TTN<1×10-2. For all three elements, we find power-law behavior with the same critical exponent =0.310.01. Although Gd3+ is an S-state ion and the dominant interaction is isotropic Heisenberg exchange, this exponent is Ising-like. This is interpreted as being due to the strong uniaxial character of the magnetic order. Our results are compared with those for other magnetoelectric materials. © 1977 The American Physical Society.