Abstract: The structures and morphometric comparisons of the pre-parasitic juveniles of the following Mermithidae (Nematoda) were studied:Romanomermis wuchangensis,Hexamermis albicans,Hexamermis agrotis,Hexamermis fengchengensis,Hexamermis sp.,and Amphimermis sp.Duncans new mutiplerangt test (P=0.05) was used to analyze the morphometric data of the juveniles.The results showed that there were some distinguished morphological differences among genera and that morphometric comparisons among species in the same genus differed significantly.The possibility of pre-parasitic juveniles be used for the identification of genera and species,the method of taxonomy and the main values of diagnosis were preliminarily studied in this thesis.