The Knobbed mutant of the silkworm, Bombyx mori, is a dermal mutant characterized as having paired knobs in specific regions of larval segments. To clarify the knob formation mechanisms, the epidermal cell proliferation and morphology of the knobbed region during the fourth instar were investigated. Mitosis of the epidermis in the non-knobbed region was mainly observed to occur at 48 hours (h) after ecdysis. Conversely, numerous mitotic cells were observed in the knobbed region not only at 48 h but also 24 h after ecdysis. Mitotic cells were also observed at 72 h after ecdysis. According to observation using the transmission electron microscope, epidermal cells at the knobbed region were considerably longer and more slender than those of the non-knobbed region. The unusual shape of the epidermal cells in the knobbed region appeared attributable to abnormally prolonged mitotic activity. In this paper, based on these morphological observations, the process of knob formation was discussed.