The class Prasinophyceae (Chlorophyta) contains some photosynthetic eukaryotlc ultraplankton species characterized by containing prasinoxanthin. The existence and abundance of these organisms can be estimated by the diagnostic pigment. We detected the unique pigments of prasinoxanthin-contalning Prasinophyceae in Jlaozhou Bay, China using high performance liquid chromatography (HPLC). This was the first finding of this kind in Chinese seas. Using the ratio of prasinoxanthin to chlorophyll a, the abundance of prasinoxanthin-containing Prasinophyceae has been calculated. The average contribution of prasinoxanthin-contalning Prasinophyceae to the chlorophyll a pool was 8.5% and 17.0% in May and August 2004 in Jiaozhou Bay, and the maximums were 25.9% and 36.3%. Size fractionated pigment analysis suggested that more than 80% of prasinoxanthin were in the fraction of 2-20 μm.According to the results of pigment and morphological analysis, the possible genera of prasinoxanthin-containing Prasinophyceae and the reasons for causing this high abundant phytoplankton in Jiaozhou Bay were discussed.This kind of phytoplankton can not be discovered in traditional biological investigation, but its contribution to the coastal ecosystem is significant enough to be studied further.