Ammonium contents of biotites from metamorphic and granitic rocks of Japan have been determined, and correlated with the 18O /16O ratios of the rocks. NH 4 contents of biotites averaged 22 ppm in granitic rocks of non-metamorphic terranes, 67 ppm in granitic rocks in the Ryoke metamorphic belt, and 279 ppm in metamorphosed sedimentary rocks of the Ryoke belt. In granitic rocks, enrichment of NH 4 in biotites is a result of the interaction between the granitic magma and surrounding sedimentary rocks. In metasedimentary rocks, the high NH 4 content in biotites is due to inheritance from original organic material in sedimentary rocks. Biotites from migmatites of the Ryoke belt contain more NH 4 (average, 475 ppm) than those from metasedimentary rocks. This suggests the existence of a metamorphic fluid or anatectic magma enriched in NH 4.