In order to examine the processes by which a living community of diatom is transferred to a dead assemblage, the transportation and sedimentation of dead diatoms in a tidal area were investigated by comparing the distribution of the living diatoms with that of the dead ones. Living cells can be clearly distinguished in total assemblages including dead ones by the double stained method using Hematoxylin and Eosin modified by Kosugi (1985a). In this paper, spatial distributions of diatoms in/on bottom sediments obtained from the lower reach of the Obitsu river, Chiba Prefecture, in the central Japan, are dis-cussed. The R-mode cluster analysis based on Jaccard's coefficent was used on living diatoms in order to examine the distributions of them and to classify into several groups. Percentages of perfectly preserved dead cells were calculated to know the degree of breakage in the process of transportation and sedimentation. Moreover, the transportation and sedimentation of dead diatoms were discussed from the view point of the relations to water current directions, surficial bottom sediments and preserved degrees of dead cells. The results are as follows: 1. The cluster analysis (Fig. 4) indicate that nine species groups of living diatoms which clearly originate in the living habitats are distinguished. 2. The dead cells found in the areas where the same species are now living are considered as autochthonous, and the other cases are defined as allochthonous. Allochthonous dead cells are dispersed to all over the study area, although the degree of dispersion differs with species (Fig. 5). 3. In the area where the sea water intrudes at high tide, dead cells of diatoms living in the lower areas are probably transported to the upper areas by tidal current. 4. The number of allochthonous dead cells per 1 cc of the bottom sediment is large in the environments where sediments are mainly made of mud and humic matter, such as tidal creeks and open type salt ponds. The small number of cells is found in the sedimentary environment made of well-sorted sand, such as foreshores, the river mouth of the Obitsu river and closed type salt ponds. 5. The percentages of perfectly preserved cells have tendency to become lower depending on the transported distance. The percentages of dead allochthonous cells are lower than those of autochthonous ones. The percentages of total dead cells are low in environments where the number of allochthonous dead cells in sediment is large.