AbstractFetal thymus colonization is initiated before the vascularization of the thymus primordium. This prevascular colonization of the fetal thymus by T‐lymphoid progenitor cells is guided by the coordination of CCR7‐ and CCR9‐mediated chemokine signals. However, the intracellular signals that mediate the prevascular migration of T‐lymphoid progenitor cells to the fetal thymus are unknown. Here we show that T‐lymphoid progenitor cells in fetal mice express two closely related CDM family molecules, DOCK2 and DOCK180. We found that the prevascular fetal thymus accumulation in vivo was significantly reduced in mice doubly deficient for DOCK2 and DOCK180 but not in mice deficient for either DOCK2 or DOCK180. Immature T‐lymphoid cells from mice doubly deficient for DOCK2 and DOCK180 were defective in their in vitro migration towards fetal thymus lobes. The T‐lymphoid progenitor cells generated in mice lacking DOCK2 and DOCK180 were capable of T‐cell development after their transfer into a fetal thymus environment, and the impaired fetal thymus colonization in mice deficient for DOCK2 and DOCK180 was not as severe as that in mice doubly deficient for CCR7 and CCR9. These results indicate that the combination of DOCK2 and DOCK180 plays a significant but not essential role in prevascular fetal thymus colonization.