Spatial and temporal variation in metazooplankton biomass size spectrum was studied from April 2001 lo December 2002, in a tropical reservoir, Liuxihe reservoir (South China). The SheldonWesl Lakel)pe size spectrum of metazooplankton (50 to 512jxm equivalent spherical diameter) was nol uniform, but showed an increase in biomass between successive size classes. Within rotifers (50jxm to I60|xm ESD), biomass decreased slightl) in size, while cladoceras (100p,m to 5I2jxm ESD) had maximum biomass concenlration per size class a(medium ranges of body size. Copepods was the most important zooplankton fraction (contributed more than 60% of total zoo-plankton biomass),and covered all 10 size classes from 50 to 512jjliti in this study. Biomass of ro[)epo(is showeci an overall increase in size. As man-made lakes, reservoirs occupy an intermediate position between rivers and natural lakes by corn billing numerous features of rivers and lakes. Due to this unique feature, metazooplankton size distribution changed s|)a-tiaily in the reservoir, and the slopes of size spectra showed a different seasonal pattern between the riveri/ie zone and the lacustrine zone. On average, small sized organisms, rotifers and nauplii, contributed a relatively more percentage biomass in the riverine zone than in the lacustrine zone, whereas adult Mesocyclops thermocyclopoids, Phyl-lodiaptomus tunguidus and their late stage copepodites contributed less percentage biomass in the riverine zone. As a result, the slope of normalized biomass spectrum was more negative in the riverine zorje than (hat in the iaruslrine zone. In llie riverine zone, slopes were more negative than -0.75 when water levt*l decreased in llie dr> seasons ami the re\erse when water level iiKTeased in flood seasons. The len\poral \arialions might he aliribuied lo llie water level flurtuation in the riverine zone. In contrast to the riverine zone, as the depth is deep enough in tht* Jacustrint* zone, anti waler level fluctuation showed no effect on the size distrihution of inetazooplankton. Size (listributimi of metazooplaiikton was relatively stable over seasons.