Abstract: Using Chlorella-containing Paramecium bursaria (CCP),recovered Chlorella-containing P.bursaria (RCCP) and Chlorella-free P.bursaria (CFP) as materials,the effect of symbiotic Chlorella on the cell size,growth,resistance to oxidation,and saccharide content of the host cells were measured.The highest density of cells in the three cultures was 1500,800,500 cells/mL,respectively,corresponding to the peak period of growth on day 14,14 and 8.CCP cells averaged 22 μm longer and 16 μm wider than CFP ones,and those of RCCP cells fell between CCP and CFP.The resistance to O2-chemiluminescence produced by self-oxidation of pyrogallol was different:when concentrations of the cytoplasm homogenate increased from 20 to 100 μL/mL,CCP increased for 67%,RCCP for 45%,CFP for 27%,and the symbiotic Chlorella for <10%.Of glucose and maltose in cytoplasm,CCP contents were 4 and 2 times higher than those in RCCP,respectively,and CFP were only 1/4 and 1/2 of RCCP.It is very likely that paramecium-hosts benefit from the symbiotic system.