Abstract: It is well established that cyanobaceria are able to produce a range of toxic compounds,of which the microcystins are the most commonly known. In recent years,investigation into the toxicity of microcystins have mainly been centred on animals. Some data are available on toxicity to terrestrial plants. Little work has,however, been done on the effects of microcystins on aquatic plants. In recent years,aquatic macrophyte has reduced rapidly due to various reasons. We investigated the effect of MC-RR(from microcystins in Dianchi Lake)on the growth and development of a submergent macrophyte Vallisneria natans. Seeds are bacteria free cultured in MS medium with MC-RR. On day 10,20 and 30,seedlings were taken photographes and seedling length measured. Results are as follows:V.natans are sensitive to microcystin-RR. At the concentration of 0.0001-10mg/L,seed germinaton rate,cotyledon length,leaf length,root length and root hairs declined in varing extent,and fresh mass of plants,the root and leaf formation were inhibited. Over the concentration of 0.01mg/L, fresh mass of plants and leaf length was significantly reduced after 30d treatment. At the concentration of 10 mg/L,the root growth and leaf formation were also inhibited significantly and the root hairs disappeared,the new leaf turned yellow. It can be concluded that MC is harmful to the plant life of Vallisneria natans.