The distribution of fibronectin (FN) was studied by immunofluorescence during gastrulation and early neurulation in avian embryos. FN was first detected under the upper layer (ul), just prior to the streak appearance. During gastrulation, FN was restricted to the basement membrane of the ul. The streak initially appeared as a protruding cell mass limited by FN strands. Later on, FN disappeared locally where the streak sank into the space under the ul. In the streak, the invaginated cells separated and migrated laterally along the FN-rich basement membrane of the ul. FN appeared in the basal surface of the definitive endoderm, when it was completed. Our results suggest that, prior to gastrulation, passive cell movements occur in the absence of FN, while during gastrulation, FN is involved in active movements. In the head, early neurulation followed the separation of the notochord from the neural plate by a FN-rich basement membrane. In contrast, in the trunk, the neural groove appeared first and remained in close apposition with the notochord and somitic mesenchyme twice as long as in the head. An intense FN staining was revealed first between the neural tube and the somite, and later on between the neural tube and the notochord.