The intermetallic compounds CeAl and PrAl a (PrAl annealed) crystallize in the Cmcm orthorhombic space group. They exhibit a metamagnetic behavior below their Néel temperatures. Both compounds have the same noncollinear magnetic structure; rare earth atoms are divided into two sublattices with different magnetization axes which make the same angle with the b axis. Inside each sublattice the arrangement of the moments is collinear and antiferromagnetic. We show that the two moment directions are essentially due to the crystal field effects which act on the rare earth ions lying in a low symmetry site.