In land ecosystems nematodes linked with plants form one of the most widely distributed groups of soil invertebrates. High species diversity, abundance and biomass of these animals indicate their functional importance in ecosystems. Information on species composition of this group, however, remains very limited, and study of these nematodes is difficult on account of their great diversity. The group includes free-living saprobes (typical and atypical saprobionts), predators and parasites (specific and non-specific). All these differ in their ecology and the different precise niches they occupy. Investigators of their diversity must thus look at different parts of plants (root, stem, seed), soil, litter, and must take into account the floristic composition of the community. Results have shown that different soil and litter strata are inhabited by different species. All potential habitats must therefore be studed to an accurate picture of species diversity. Examination of species composition of nematods in 40 plots in undisturbed and secondary ecosystems in the Carpathian region showed that maximum species diversity in each altitudinal zone was to be found in undisturbed ecosystems. Secondary ecosystens possess a depauperate species composition of nematodes, but can be inhabited by species not encountered in undisturbed communities. It is thus essential to examine both undisturbed and secondary communities to reveal the full biodiversity of any area under study.