Elastic neutron scattering has been used to study the magnetic structure of erbium in an applied magnetic field of up to 2.8 T along the c axis. There are three major types of ordered structure: the cone phase, an intermediate-temperature phase with moments modulated both along the c axis and perpendicular to the c axis, and a phase at high temperatures where only the c component of the moment is ordered. Within these phases the modulation of the moments may lock in to the lattice. The field tends to stabilize the cone phase and to destabilize the intermediate phase. A lock-in structure with q=(1/4)c * appears in the cone phase for fields above 1