Abstract: From January,1987 to January,1988,the ecological aspects of Ergasilus hemibagri (Cyclopoida),which is parasitic on the gill of the fish--Mystus macropterus in the Fujiang River of Hechuan County,China,have been studied.The conclusions can be drawn as following:the parasite can reproduce above 16℃ and it possesses high reproduction and efficiently infective power about 25℃;the infections of the parasite varied seasonally as some factors changed,such as temperature,water quality,current velocity,the reproduction and infective power of the parasite etc;the frequency distribution of the parasite is negative binomial distribution,which is fit,when tested with chi square test;is is probable that the host produces resistance against the parasite when it is infected by the parasite,and the resistance increases with the increase of the number of infection and intensity;in natural environment,the factors influencing the population growth of the parasite are density of independent factors and immunological responses generated by the host.