The crystallization ofBombyx mori L.silk fibroin was carried out from a regenerated solution maintained in contact with air under a steady-state flow. The nucleation occurs at first close to the solution surface after a few seconds of stirring and then a fibrillar crystal propagates. The nucleation was considered to occur by the flow-induced, one-dimensional orientation of silk fibroin molecules which were adsorbed to the solution surface. The yield of precipitate after stirring for 2 hr was influenced by the temperature and concentration at low shear rate (below 200 sec−1), while it was not influenced at high shear rate (1000 sec−1), and was above 80%. Electron microscopy, X-ray diffraction, and DSC thermograms show that the precipitates consist of fine fibers of about 0.2 μm diameter and crystals which have the β-structure and a low orientation.