Brachionus urceus (Linnaeus) isolated from Lake Taihu was incubated at a density of 100 individual•L-1 for 24 h with sufficient Scenedesmus obliquus as food. After 24 h, the culture media was filtered through a 0.10 μm membrane filter and used as test water, which contained some infochemicals exuded by B. urceus. In the treatment, 20 ml B. urceus culture media filtrate was added to an 80-ml suspension of S. obliquus, while in the control, 20 ml BG-11 medium was added. The experiment was run in triplicate for 7 days at 25℃ in photoperiod of 12L∶12D. The results showed that exposure to the B. urceus culture media filtrate induced colony formation and promoted the mean number of cells per individual in S. obliquus population significantly (P<0.05). Colony formation in S. obliquus population induced by B. urceus increased their resistance and thus reduced the risk of being grazed, which can be viewed as a kind of inducing defense, and also further demonstrated the morphological responses of Scenedesmus to herbivorous zooplankton is rather widespread. No significant difference in growth rate was detected between unicellular and colonial S. obliquus, suggesting that the cost of inducible defense of colony formation did not reflect on these indices. The contents of soluble extracellular polysaccharides and bounded extracellular polysaccharides in S. obliquus increased significantly after colony formation, whereas the content of intracellular polysaccharides decreased slightly. The total content of polysaccharides in the treatment was obviously higher than that in the control. These results indicate that rotifer culture media filtrate stimulate S. obliquus to increase the synthesis of polysaccharides that are primarily secreted from the cells and may act as the cementing material which aggregates the cells firmly together to form colonies. Such large morphologies of S. obliquus can effectively deter the alga from grazing by small zooplankton, like B. urceus, and may aid in the survival of S. obliquus in nature when confronted by rotifers.