Abstract: The larva of Eustrongylides sp.is mainly found in the cavity of finless eel Monopterus albus.It is wrapped in a capsule that attaches on gut and mesentery etc.Prevalence and intensity have a tendency of increases with the increases of host body length,but it has a fluctuation.Mean intensity drop first and increases follow in finless eel of body length 350—400mm,prevalence falls in finless eel of body length more than 450mm.The change is probably related to the feeding differentiation of finless eel and the immune response of host fish infected by the nematode(larva).Abundance increases lineally with the increases in host body length.The relationship between mean abundance(M)of parasite population and body length(L)of host fish are:M=-1.0760+0.0085L.The end of frequency distribution of the nematode population in host population lengthens with the increases of host body length.The range of the nematode(larva)body length are 10—70mm,but their body length are mostly 40—60mm in host population,in April to May.The over-dispersion of parasite population in host population was observed,and the degree of the aggregation increases with the increase in mean abundance.