In the Sanbagawa metamorphic belt in the northwestern part of the Kii Peninsula, occurrences of biotite and its altered products are newly recognized. Pelitic schists in the area within the previously accepted garnet zone occasionally contain biotite and/or a mineral, which is optically almost identical to biotite. The latter mineral has variable K₂O contents less than 6 wt%, and has similar stoichiometry to biofite except low-K content. EPMA and XRD analyses suggest that the mineral is weathered biotite, which contains interlayered vermiculite. Taking the localities of such altered biotite-bearing pelitic schists into consideration, the biotite zone is newly established as a narrow region along the east-west trending axis of the Dainichi synform. The Fe-Mg partition coefficients for garnet-chlorite pairs in the pelitic schists of biotite zone indicate that the biotite zone in the Kii Peninsula corresponds to the lower-grade part of the albite-biotite zone in central Shikoku.