Abstract: Colacium sideropus Skuja is one of main algal populations in Donghu lake,Wuhan during winter and spring. It mainly attaches on cladocerca and Copepoda (belonging to Crustaea).Its growth peak appears in March(late winter and early spring). The population growth corresponds to the J-shaped growth form. The interaction between C.sideropus and its hosts:The interaction between them is mostly commensalism, i. e., to the benefit of C.sideropus; but doesn't exert any influence upon its hosts. So both population densities (N) of C. sideropus and host's densities (Na) have close positive correlation which may be described as N= 0.0858e0.0528Na(r=0.921,pC. sideropus attaching on its hosts is huge, the interaction between them is amensalism, i.e., host's morement will become slow and go so far as to sink (even die). The influence of host's exuviation on C.sideropus: The mean attachment amount (m) and the attachment amount (B) in relation to interval time (T) of host's exuviation may be positive within fixed time range: m=aebT and B= aebT; or negative beyond the time: m=aT-b) and B=aT-b) (a and b are costants, see figs. 2 and 3). But the attachment rate (R) in relation to interval time (T) of host's exuviation is only positive:R=22.95+30.40lnT(r=0.834,pD.hyalina; R=32.50+37.00lnT (r=0.729, 0.05>p>0.01, fig.4) in C. vicinus Selectively of C. sideropus to its hosts: The attachment ability of C. sideropus is the strongest to Daphina hyalina (Cladocerca); second to Cyclops vicinus (Copepoda) and the weakest to Neutrodiaptomus incongruens (Copepoda) (see table 1). The water temperature exerts an influence on the population of C. sideropus: The correlation between the population densities (N) and the water temperatures (t) may be positive within 5-12℃:N=0.309e0.624t)(r=0.914,pN=0.0000617e190·2/t(r=0.941,pC. sideropus is the fastest when the water temperature is 12℃. So 12℃ may be the most suitable temperature for the growth of C.sideropus. Samely, the correlation between the mean attachment amounts(m)and the water temperatures (t) may also be positive within fixed range of the water temperature: m=bt-a; or negative beyond the range: m=aeb/t(a and b are constants, see fig. 6). But the attachment rates (R) of C.sideropus in relation to the water temperatures (t) is only negative (see fig. 7): R=39.37ln(30-t)-20.89(r=0.933, pD.hyalina; R=40.95ln(30-t)-30.61(r=0.939,p0.01),in C. vicinus; R=31.54ln(30-t)-44.74(r=0.843,pN.incongruens. The influence of trasparency: The population densities (N) of C.sideropus in relation to the transparencies (d) is positive. It may be described as: N=0.020 e0.037d (r=0.838,pC.siderppus because the lake is eutrophic.(fig.9). The population densities (N) of C. sideropus in relation to calcium concentrations (S) may be described as N=0.00254e0.178S (r=0.816, 0.05>p>0.01,fig. 10). Multiple regression analysis of synthetic influence on ecological factors: It is very remarkable that the population growth (N) of C. sideropus in relation to the densities (Na) of hosts, trasparencies (d), water temperatures (t) and calcium concentrations (S) is analysed by multiple regression. The multiple regression equation may be described as N=1.208Na+0.698d+5.584t+2.9425-357.957 (r=0.853,df=11,k=4,p<0.01)