Transforming growth factor beta (TGF beta) inhibits proliferation and promotes the migration of primordial germ cells (PGCs) towards explants of gonadal ridges in vitro. However, its effects in vivo are still unclear. Here, we analyzed the behavior of PGCs in embryos lacking TGF beta signaling via the type I receptor ALK5. TGF beta in vivo was neither a chemoattractant for PGCs, nor did it affect their proliferation during migration towards the gonadal ridges up to embryonic day (E) 10. Unexpectedly, the absence of TGF beta signaling in fact resulted in significant facilitation of PGC migration out of the hindgut, due to the reduced deposition of collagen type I surrounding the gut of Alk5-deficient mutant embryos. Migratory PGCs adhere strongly to collagen; therefore, reduced collagen type I along the gut may result in reduced adhesion, facilitating migration into the dorsal mesenterium and gonadal ridges. Our results provide new evidence for the role of TGF beta signaling in migration of PGCs in vivo distinct from that described previously.