Disordered sericites from the western part of Japan were found to have ammonium ions which are believed to be located in the interlayer sites. Evidence for the presence of NH4+ ions was obtained from chemical, infra-red, and weight-loss analyses. X-ray examination of a large number of sericites revealed that these minerals have basal spacings ranging from 10.0 Å to 10.5 Å, and that disordering in the layer-stacking increases with increasing basal spacing. The x-ray diffraction patterns of these minerals showed similarities to the 2M1-, lM-,and lMd-types, but detailed structural analyses have not been made, by which the polytype could be definitely identified. In this investigation a tentative classification has been made based on the average basal spacings; 10.0 Å (2M1-type), 10.05 Å(1M), 10.2 Å(1M), and 10.5 Å(lMd), respectively.