The distribution and localization of fibronectin (FN) on the migratory pathway of primordial germ cells (PGCs) in mouse embryos were studied immunocytochemically at the light and electron microscopic levels. In embryos 9.5 to 11.0 days of gestation, the dorsal mesentery as the final region through which PGCs migrate was rich in FN. At this stage, migrating PGCs often showed amoeboid features with pseudopods in contact with neighboring mesentery (mesenchymal) cells. With the electron microscope, the reaction product to FN was visualized on the surfaces of somatic cells and of PGC pseudopods and at the site of contact between PGCs and somatic cells. Abundant extracellular FN was also found, probably binding with the extracellular matrices. By 11.5 to 12.0 days, when PGCs had arrived in the gonadal anlage, FN reaction had weakened or disappeared in the dorsal mesentery. Thus, the results suggest that FN plays a significant role in the migration of PGCs at least in the last portion of the migratory pathway.