The present paper studies the seasonal change, quantitative variation and distributional patterns of planktonic Ostracoda along the western coast of the Taiwan Straits and their relationship with various environmental factors, especially those of the movement, intensification and decay and interaction of different water systems.Except for some study of Ostracqda in the Straits[14], ours is based on the samples of planktonic Ostracoda and field hydrological and chemical data collected during the two comprehensive investigations (Oct. 1961-Sept. 1962 (no data for July, 1962) 23°16′-24°15′ N, 117°16′-118°13′E and March, 1963-Feb. 1964, 24°34′-25°43′ N, 118°46′-119°52′ E).The total amounts of planktonic Ostracoda along the western coast of Taiwan Straits show evident seasonal variations with greater amounts in summer and autumn. The yearly maximum appeared in October and subpeaks in May and August. The quantity of the animal was lowest from January to April.There are all together 17 species The present paper studies the seasonal change, quantitative variation and distributional patterns of planktonic Ostracoda along the western coast of the Taiwan Straits and their relationship with various environmental factors, especially those of the movement, intensification and decay and interaction of different water systems. Except for some study of Ostracqda in the Straits [14], ours is based on the samples of planktonic Ostracoda and field hydrological and chemical data collected during the two comprehensive investigations (Oct. 1961-Sept. 1962 (no data for July, 1962) 23 ° 16’-24 ° 15’N, 117 ° 16’-118 ° 13’E and March, 1963-Feb. 1964, 24 ° 34’-25 ° 43’N, 118 ° 46’-119 ° 52’E) The total amounts of planktonic Ostracoda along the western coast of Taiwan Straits show evident seasonal variations with greater amounts in summer and autumn. The yearly maximum occurred in October and subpeaks in May and August. The quantity of the animal was lowest from January to April .There ar e all together 17 species