In order to clarify the microflora of the lower course of the Han River and the effect of the marine water on the Han River, the study was carried out at Paldang, Kwangjang, Noryangjin, and Haengju for 4 months (from May till September in 1965). The results obtained are as follows: 1). water temperature, transparncy, light intensity. pH, silicate, and salinity were determined as environmental conditions. 2). Samples collected from 4 stations were identified and classified by Engler`s classification system. It resulted in 4 Phyla, 3 Classes, 13 Orders, 25 Families, 61 Genera, 155 Species, and 16 Varieties. The total numbers of phytoplanktons identifed are 171. Of the number, 106 species and 12 varieties were recorded in this paper for the frist time. 3). A comparative observation on the distribtuion of marine phytoplanktons and salinity in the Han River showed a fact that the Han Riber was biologically affected in the middle portion between Noryangjin and Haengju by marine water. Furthermore, the salinity determined at Haengjuduring a day(at high tide, September 26. 1965) supported the above fact. 4). Four species among the identified species are saprobic planktons and it is regarded as the forms derived from Chunggyechon.