Cyclomorphosis is a common phenomenon which helps rotifers adapt to aquatic environments. To examine the mechanism and the ecological significance of cyclomorphosis of rotifers, the occurrence and the morphometric characteristics (body length, body width, and lengths of antero-median spine, left and right antero-lateral spines, and left and right posterior spines) of Brachionus forficula in Lake Jinghu and Lake Tingtang were investigated for one year, and the relationships between morphometric parameters and ecological factors (water temperature, pH, total Chl-a concentration, Chl-a concentration in water filtered through 25 μm net, SD readings, DO, N-NH^-₄ concentration, and densities of Asplanchna, all rotifers except Asplanchna, cladocera and copepod) were regressively analyzed. The results showed that in Lake Jinghu, B. forficula occurred from July to October, and all the individuals belonged to short posterior-spined morphotype. Except for the antero-lateral spine length, all the other morphometric parameters of the rotifers at the two sampling stations varied markedly with season. The differences in morphometric parameters of the rotifers between the two sampling stations also differed with season. All the morphometric parameters of the rotifers increased with increasing pH, but decreased with increasing total Chl-a concentration and Chl-a concentration in water filtered through 25 μm net. Body length and width of the rotifers both increased with increasing Asplanchna density. In Lake Tingtang, B. forficula individuals with short posterior spines occurred from June to October, but those with long posterior spines occurred only from June to August. When the two morphotypes occurred simultaneously, the proportion of the individuals with long posterior spines in the population was always higher than that of the individuals with short posterior spines, and correlated positively with water temperature, total Chl-a concentration in water and densities of cladocera, copepod and Asplanchna. All the morphometric parameters of the individuals with short posterior spines at the two sampling stations varied markedly with season. Only the lengths of the antero-lateral spine and the posterior spine at Station Ⅰ, and the body length and width, and the posterior spine length of the individuals with long posterior spines at Station Ⅱ varied remarkably with season. At the two stations, the body length and width, and the antero-lateral and posterior spine lengths of the individuals with long posterior spines were all significantly longer than those of the individuals with short posterior spines. The differences in morphometric parameters of the individuals with an identical morphotype between the two sampling stations differed with the season. The lengths of the antero-lateral spine and the posterior spine of individuals with short posterior spines were both positively correlated with total Chl-a concentration, and the latter was also positively correlated with Chl-a concentration in water filtered through 25 μm net and the density of cladocera. The posterior spine length of the individuals with long posterior spines was positively correlated with copepod density, and the antero-median spine length was negatively correlated with water temperature, DO and Asplanchna density. The posterior spine length of B. forficula in both the two lakes was positively correlated with the body length, indicating an absence of allometric growth of the posterior spine.