The central and southern parts of the Bohai Sea was investigated by the authors during a cruise by the R. V. Dongfanghong No. 2 in September and October, 1998 and April and May, 1999. Except for larval zooplankton, 53 zooplankton species were identified. Labidocera euchaeta, and Sagitta crassa dominated in autumn, 1998. While Calanus sinicus, Centropages mcmurrichi dominated in spring, 1999. Copepods accounted for 32%-83% of the total mesozooplankton abundance in the 4 investigations. S. crassa accounted for 33%-38% of the total abundance in autumn. A medusa, Rathdea Octopunctata, occurred in abundance at some stations in spring, 1999. The maximum abundance of C. sinicus was 249 and 1442 ind./m3 in autumn and spring respectively. The maximum abundance of L. euchaeta and S. crassa in autumn was 494 and 283 ind./m3, respectively. In the spring, the abundance of C. mcmurrichi reached its maximum of 2605 ind./m3. The biomass (wet weight in mg/m3) of zooplankton ranged from 26-488 ind./m3 (143 ind./m3 on average) in Sep 1998, 35-477 ind./m3 (167 ind./m3 on average at 30 stations) in Oct 1998, 36-1257 ind./m3 (293 ind./m3 on average) in Apr 1999 and 88~916 ind./m3 (321 ind./m3,on average) in May 1999.