Species composition, abundance and diversity of the Planktonic Ostracoda were investigated during four cruises conducted in the Beibu Gulf, between Feb 1998 and May 1999. A total of 18 species of planktonic Ostracoda were identified, including Cypridina nana, Cypridina dentata, Cypridina acuminata, Cypridina nami, Euconchoecia aculeata, Euconchoecia elongata, Euconchoecia maimai, and other taxa. Species composition was relatively simple, with four species occurring year round, nine species only appearing in one certain season, and five species appearing in several seasons. The maximum number of Ostracod species (11) was recorded during both summer and winter, while a minimum of 6 species were present in autumn. Planktonic Ostracoda in the gulf were divided into 3 ecological types: (1) a hyposaline and warm-water group, (2) a eurythermal and eurysaline group, and (3) a hyperthermal and hysaline group. The warm water species such as Euconchoecia aculeate and Cypridina acuminate were the dominant Ostracod species in the gulf, where overall Ostracoda density ranged from 0.02 to 51.58 ind./m³ and averaged 1.22 ind./m³. The highest average value (2.73 ind./m³) was found in spring. In the northwest region of the gulf, the assemblage consisted of Euconchoecia aculeate, and in summer the dominant species shifted to Cypridina nami in the middle of the gulf. In autumn and winter, although Ostracoda density was low, two assemblages with relatively high abundances could be found in the southern and northern gulf. The density was gradually lower toward the center from the southern and northern gulf. Seasonal variations in horizontal distribution of biomass showed similar patterns as those in density mentioned above. The diversity threshold of Ostracoda averaged 0.329 in the gulf. The mouth and the head regions of the gulf had the maximum and minimum diversity index values, respectively. A trend toward decreasing diversity was observed along the gulf northwestward from the mouth. Fuzzy evaluation measures indicated that the diversity in the gulf remained low (class 1 level) during all seasons. Diversity appeared to be relatively higher in some regions in which the water mass was strongly influenced by the open sea and had complex structures. This paper also discusses how the species composition, abundance, and diversity of planktonic Ostracoda are influenced by environmental factors (e.g., aquatic system, water mass, water temperature, salinity) in the Beibu Gulf.