The crystallization behaviour of Fe65Co18B16Si1 ribbons obtained by planar flow casting under various conditions is studied by differential scanning calorimetry, magnetic measurements, X-ray diffraction and transmission electron microscopy. Microstructural analysis show that the thick ribbons contain a eutectic superficial structure in their free side while the thin ribbons are amorphous. The final products of crystallization are b.c.c. α-(Fe,Co) and orthorhombic (Fe,Co)3B. Crystallization occurs in two stages, the first one being favoured and the second one being retarded for the thicker ribbons. The coercive field of the thick ribbons increases with respect to that of the amorphous ribbons as a result of the presence of quenched-in crystals. © 1991.