The crystal structure and magnetic ordering of the novel orthorhombic compound ErGe 3 of RGe 3 -type structure (Cmcm) was studied by neutron powder diffraction and magnetic measurements. ErGe 3 orders antiferromagnetically below TN = 7 K with a collinear antiferromagnetic moment arrangement with the magnetic space group C112 1 /m (P2 1 /m ) (Sh 1 1 5 3 ). The Er magnetic moments are confined to the (a,b) plane making an angle of 15° with the a axis. The ordered magnetic moment value at 1.5 K is 8.6 μ B , per Er atom. Field-dependent magnetic measurements at 4.2 K show that ErGe 3 is metamagnetic, and transforms to a ferromagnetic state with the same ordered magnetic moment value in fields larger than about 2 T.