We performed atomic force microscopy measurements on fibroin molecules from the domestic silkworm, Bombyx mori. At low concentrations, we could observe single protein molecules. The shape of the observed molecules is a rod with long smaller chains extending from the ends. The size of the rod is 60 nm in length and 15 nm in width. At high concentrations, we observed long threadlike aggregates of fibroin molecules, their chains entangled with one another. These results provide insight into the microscopic mechanism of silk-fiber formation.