The metal-insulator transition in the orthorhombic perovskites PrNi03 and NdNi03 is accompanied by the sudden 3D magnetic ordering of the Ni ions (, uNi« 0.9¡xB). The magnetic ground state, described by a commensurate Ar = (1/2,0,1/2) spin density wave, is unprecedented in a perovskite structure. The coexistence of F and AF interactions suggests the existence of an orbital superlattice. This results from the breakdown of the degeneracy of the Ni111 (tig eg) state due to electronic correlations. The gap is probably of charge-transfer type rather than originated by the exchange interactions. © 1992 IOP Publishing Ltd.