Abstract: The bacterial viviform state is an important ecological problem which is relative to water quality accessment and environmental protection in natural water bodies. Though the reasons why it was formed are unclear and complicated,the direct effects came from the environmental stress factors including nutrient starvation,temperature changes and algal toxins stress etc. The rates of bacterial viviform states in different samping stations of Xili reservoir were investigated,the results showed: the bacteral total number was similar between the station of Dongjiang river entrance and the internal reseroir,but the rates of bacterial viviform state were lower in Dongjiang river entrance. It was deduced that environmental stress factors (such as high light intensity,water pollution and oxygen stress etc.)induced bacterial cell tybe transformation from culturable state to viviform cell. It was found that there were some relationship between blue-green bloom and bacterial viviform state in natural water bodies by studying on eutrophic water bodies in Lake Wenshanhu,Lake Xili and Lake Kinneret etc. The algal toxin is an important environmental factor in water bodies of blue-green bloom,the further experiments which studied on the influence of microcystins-LR on the viviform state of bacterium was done by using artificially induced microcystin-LR concentration gradients. The results showed microcystin-LR had no obvious effect on the aquatic bacterial population,but it could stimulated the bacterial cell transformation from viviform state to culturable state. It indicates that there is the direct relationship between microcystin-LR and the viviform state of bacterium in natural bodies of water. The further research on the molecular mechanism of microcystin-LR stimulating cell type transformation is onging.