The magnetic structures of Pr and Nd single crystals have been studied by neutron diffraction with applied magnetic fields up to 46 kOe. No spontaneous magnetic order has been detected in Pr, but a magnetic field applied perpendicular to the c-axis induces a large ferromagnetic moment. The induced moment with the field parallel to the c axis is much smaller, indicating a highly anisotropic susceptibility. In Nd the magnetic field produces substantial modifications of the periodic magnetic structure