The molecular mechanisms that direct the migration of early T lymphocyte progenitors to the thymus are unknown. We show here that P-selectin is expressed by thymic endothelium and that lymphoid progenitors in bone marrow and thymus bind P-selectin. Parabiosis, competitive thymus reconstitution and short-term homing assays indicated that P-selectin and its ligand PSGL-1 are functionally important components of the thymic homing process. Accordingly, thymi of mice lacking PSGL-1 contained fewer early thymic progenitors and had increased empty niches for prothymocytes compared with wild-type mice. Furthermore, the number of resident thymic progenitors controls thymic expression of P-selectin, suggesting that regulation of P-selectin expression by a thymic 'niche occupancy sensor' may be used to direct progenitor access.