A large-scale bloom occurred from May to June in 2011 in sea area near Qinhuangdao of the Bohai Sea, leading to huge damage of the scallop culture industry. Similar blooms have been observed in this region for three years. The causative species of the bloom, which dominated the phytoplankton community with the maximum cell density around 10 9 cell/L, could not be identifi ed with morphological features due to the small cell size (~2 m m). A pigment analytical method was then adopted to analyze the pigment profi le of the phytoplankton samples collected from the blooming sea area. It was found that pico-sized (<2 m m), nano-sized (2-20 m m), and bulk phytoplankton samples had similar pigment profi le, representing the pigment signature of the bloom-causative species. The major pigments detected included 19-butanoyloxyfucoxanthin (But-fuco), fucoxanthin (Fuco), diadinoxanthin (Diad) and chlorophyll a (Chl a ), and high content of But-fuco was the most signifi cant characteristics of the phytoplankton samples. Based on the pigment composition and content, the bloom-causative species could be tentatively identifi ed as pelagophyte, "type 8" group of haptophyte, or silicofl agellate. Some unique features of the bloom, such as the extremely high cell density, small-sized and But-fuco containing cells, occurring in early summer, and the feeding-cessation effects on scallops, suggest it be a "brown tide" event similar to those reported in the east coast of the United States of America. The recurrent "brown tide" events and their dramatic impacts on the shellfi sh mariculture industry in Qinhuangdao need close attention in the coming years.