Concentration of ammonium in biotites from Precambrian rocks in Finland has been determined under an assumption that ammonium in biotites of metasediments may be useful as a chemical fossil. Biotites from the Svecokarelian metasediments which were originally products of sedimentation about 2400-1900 Ma ago, have high ammonium contents (hundreds ppm). While biotites from the Postsvecokarelian igneous rocks (rapakivi granite, 1650-1700 Ma) and the Svecokarelidic plutonic rocks (1800-1900 Ma) have low ammonium contents (tens ppm). The results suggest that life activity was predominant during the sedimentation of Svecokarelian strata. This suggestion is consistent with a fact that valid signs of ancient life have been found from dolomites and graywacke-slates of the Svecokarelides. Biotites from the Presvecokarelidic schists of more than 2600-2800 Ma old have only tens ppm of ammonium. This value is in the same order as those of biotites from the Postsvecokarelian igneous rocks and the Svecokarelidic plutonic rocks. The results suggest that there was no life or less activity of life during the sedimentation of the original rocks of the schists. This is also consistent with a report that no evidence of fossils has been known in the Presvecokarelidic rocks. From these data, it is concluded that ammonium in biotites of metasediments is a useful chemical fossil in discussing on appearence and evolution of life in Precambrian time.