Abstract: The THird-stage larva of Contracaecum rudolphii is found in the cavity of Gymnocyprii przewalskii przewalskii. It is wrapped in a min capsule that attaches on gut and mesentery. Its mean density increases with the increase of host body length in a power function. The over-dispersion of parasite population in host population was found, and the degree of the aggregation increases with the increase mean density. This spatial pattern is probably related to the heterogeneous distribution of the first intermediate host of parasite and the food division of the fish host (G.przewalskii przewalskii) and can not be described with mathematical model. There is a significant negative relationship between the prevalence and the negative binomial distribution parameter k, which means the rising of prevalence may result in the decline of aggregation degree.