The author has had an experiment on the changes by death, of the vegetative Plankton (Diatoms) in the organs of drowned bodies at spring and autumn, and has acquired the following results: 1) There were 11 diatoms from the water of the river which has been had for this experiment. Among these 11, 8 diatoms such as Cymbella naviculiformis, Cymbella parva, Navicula Placentula, Navicula radiosa, Nitzschia filiformis, Melosira islandica, Cyclotella comta and Rhopalodia gibba were found in the lung on the 20th day after death and in the femur on 60th day. 2) There were 2～4 among 11 kinds of diatoms which were found in the heart, liver and kidney on the 20th day after death, the utmost limited time for use. 3) The diatoms which has been considered in the strength for resisting to decomposition were Cymbella parva, Melosira islandica, Cyclotella comta and Rhopalodia gibba. 4) From the results of these observations, the author has acquired the knowledges, which discovered the diatoms from the organs such as lung, heart, liver, kidney and femur of drowned bodies as far as they can be taken (used). This is supposed to be the basis in determining whether drowning is the cause of death.