Marine Public Welfare Industry Research Project [201005012]; National Natural Science Foundation of China [41276133, J1210050]; State Oceanic Administration Project [530-03-02-02-03]; State Oceanic Administration of the People's Republic of China [908-01-ST09] ; The species composition and abundance of the planktonic Hyperiidea and their relationship with environmental factors (depth, temperature, salinity, dissolved oxygen, turbidity and chlorophyll a concentration) were analyzed based on 4 seasonal investigations in the Beibu Gulf between 2006 and 2007. Hyperiids were represented by 82 species, 5 of which had not previously been recorded in the South China Sea. The hyperiid communities were dominated by Lestrigonus bengalensis year-round, Hyperioides sibaginis was dominant in spring, summer and autumn, and Tullbergella cuspidata became dominant in spring. Both species richness and the Shannon-Wiener diversity index increased from north to south, and from nearshore to offshore. Biodiversity was higher in some regions where the water mass was strongly influenced by the open sea or the lower salinity water off the northern sea shelf. Results of partial correlation ana lysis showed that species richness was positively related to depth at all times, and negatively related to temperature only in summer and winter. TWINSPAN classification indicated that the Hyperiidea community was divided into 2 groups in spring and winter ( a widely distributed group and a pelagic warm-water group), and 3 groups in summer and autumn. A clear distinction between euryhaline and pelagic species can be discerned, based on different geographical distributions of water masses.