The diagnostic usefulness of the morphological characters of the metacercariae of two similar species of the genus Diplostomum, D. paracaudum and D. pseudospathaceum, is studied. The data are based on 203 specimens of D. paracaudum and of 153 D. pseudospathaceum from fish infected experimentally. The variability of 14 morphometric features and eight indices is analysed. It appears that no single feature or index can provide a classification (discrimination) rule with a sufficiently small percentage of misclassification. In order to increase the discriminant power, a technique based on the bootstrap method is used, which, combined with a stepwise discriminant analysis, leads to the selection of five metric features. A linear discriminant function, L, obtained for selected characters separates the two species better than any single feature. It also allows each specimen to be classified as D. paracaudum if the value of the function L is positive and as D. pseudospathaceum if negative. The accuracy of this procedure is in excess of 90%.