The C. elegans Q neuroblasts and their descendants migrate along the anteroposterior (A/P) body axis to positions that are not associated with any obvious landmarks. We find that a novel protein, MIG-13, is required to position these cells correctly. MIG-13 is a transmembrane protein whose expression is restricted to the anterior and central body regions by Hox gene activity. MIG-13 functions non-cell autonomously within these regions to promote migration toward the anterior: loss of mig-13 activity shifts the Q descendants toward the posterior, whereas increasing the level of MIG-13 shifts them anteriorly in a dose-dependent manner. Our findings suggest that MIG-13 is a component of a global A/P migration system, and that the level of MIG-13 determines where along the body axis these migrating cells stop.