In order to understand the dynamics of the phytoplankton diatoms, the present study revealed species composition, fluctuation of phytoplankton standing crops, variation of the dominant species, and the causative organisms of red-tide, charateristics of red-tide due to Skeletonema costatum during the period from January 1990 to December 1991 in Inchon Dock of Korea. During these studies, phytoplankton diatoms were identified 99 taxa, representing 2 order 4 suborders 14 families 2 subfamilies 47 genera 93 species 1 variety 1 form and 4 unknown species. Phytoplankton standing crops varied from a minimum of 3.3×10 exp (3)cells/c (St. 1-B, Sept. 1991) to a maximum of 1.2×10 exp (7)cells/^2+ (St. 1-S, Aug. 1990). The first and the second dominant species were Skeletonema costatum, Paralia sulcata, Licmophora abbreviata, Chaetoceros curvisetus, C. lorenzianus, Eucampia zodiacus, Achnanthes brevipes, Asterionellopsis gracialis and Melosira borreri. The causative organisms of red -tide occurred a total number of 33 taxa and showed high frequency of Skeletonema costatum and Cylundrotheca closterium. Some environmental factors, water temperature, NO_3-N, NH_4-N, and SiO_2-Si had predominantly influenced on Skeletonema costatum red-tide.