Abstract: The plerocercoids of Ligula sp. are found in Gymnocypris przewalskii przewalskii in bodylength less than 220mm in Qinghai Lake.The over-disperion of the cestode population in thefish population was observed.The degree of the overdispersion decreases with the increase ofthe mean density.The crowding effect with the growth of cestode in the cavity of the fishmight have resulted in the death of the fish.With fish less than 120mm in body length,Thedecrease may result from density-dependent mortality; where as in fish between 120-220mmin body length,the degree of aggregation increases with the decrease of the mean density onaccount of either inversely density-dependent mortality or all-or-none type mortality.