Abstract: A parameter,index of aggregation (A),was coined in order to quantify the degree of aggregation of biological populations.This index was used to analyze the relationship between aggregation and population dynamics of Dactylogyrus vaginulatus Zhang et Niu,1966 on Hypophthalmichthys molitrix,in company with variance to mean ratio and the parameter k in negative binomial distribution.Result indicated the degree of aggregation of D.vaginulatus in host population decreased as the prevalence and the abundance increased.As a measurement,the parameter A proves to be more reliable than k,and k better than variance to mean ratio.Analyses also pointed that the power (b) in Taylors equation,which has been used widely to study the spatial pattern of population distributions,is not constant,and the death of hosts caused by parasites may be responsible for its fluctuation.The successful use of Taylors equation in biological population may come from the demographic stochasticity.