Sequence analysis reveals the presence of several haplotypes of ITS rDNA in some nematode species. Such polymorphic state of this domain provokes technical problems as ITS rDNA PCRproducts can not be directly sequenced. In such cases there is a need of more laborious and costly cloning techniques, with consecutive sequencing of separate colonies. Comparative study of ITS rDNA polymorphism was performed for three nematode species; soil-inhabiting entomopathogenic Steinernema feltiae, intestinal parasites of ruminants Haemonchus contortus and Dirofilaria immitis, parasitic in dog heart. Single haplotype can be only found in some isolates of S. feltiae, widely distributed in Holarctic (e.g. S. feltiae from Khosrov National Park in Armenia, or S. feltiae from St. Bernard pass). The individuals of other isolates of S. feltiae (e.g. Belgium- Merelbeke ‘Va') harbour two haplotypes of ITS rDNA, which differ by the presence/absence of 10 bp insert. An analysis throughout this species revealed that haplotypes with this insert are more common in Europe, and without that – in Asia. The study of intestinal H. contortus collected from domestic goats in Mongolia revealed 4 haplotypes of ITS rDNA. One of these was nearly identical (1 bp difference) with the sequence of H. contortus obtained from the sheep slaughtered in Inner Mongolia (China). Four haplotypes of ITS rDNA were discovered in a single specimen of D. immitis. All these were quite different from other ITS rDNA haplotypes of D. immitis, demonstrating only remote resemblance with the nematodes of this species from Taiwan. Phylogenetic links between the ITS rDNA haplotypes of the same nematode species reveal the relationships between the geographically remote populations. Such data can be used also as an example of intraspecific variability of ITS rDNA in these nematodes.