Although it is well known that essentially all peripheral T cells are derived from bone marrow progenitors that mature in the thymus, the mechanism whereby thymocytes gain access to peripheral compartments is obscure. We have learned that this process is sensitive to pertussis toxin (PT). Transgenic ict‐PT mice were generated which express the catalytic subunit of P‐T in all thymocytes. In a previous study we observed that T cell receptor signaling is unimpaired in these cells despite the virtual elimination of their G₁ protein signal transduction elements through endogenous PT activity. Here we demonstrate that mature T lineage cells accumulate in ick‐PT thymuses and fail to populate peripheral lymphoid organs. The accumulating cells closely resemble normal peripheral T lymphocytes with respect to cell surface phenotype and responses to allogeneic spleen cells, yet perform poorly in in vivo homing assays. This migratory defect does not result from deficient expression of common homing receptors or alterations in intracellular cAMP concentrations. Based on these results, we propose that a novel PT‐sensitive signaling pathway, almost certainly involving a G₁ protein, is required for thymocyte emigration.