The study of biological diversity at different levels of organization of the biota is one of the major challenges of modern ecology, ultimately aimed at the conservation and sustainable use of natural biological systems under dynamic conditions of anthropogenic pressure. In maintaining the structure of communities of vertebrate animals, including their biodiversity, the parasites have an important role through the regulation of host population number and prevention of alien elements penetration (Beklemishev, 1956). The variety of parasites, their life cycles and parasitic relationships, their high fecundity make them very flexible and quickly responsive mechanism for ecosystems’ stability. The evidence of parasitic organisms significantly increases the reliability of the biodiversity estimation and allows more accurately to determine the nature of dynamic processes in ecosystems (lebedeva et al., 2002). The aim of the present work is the comparative assessment of helminth biological diversity in apodemus sylvaticus (lineus, 1758) from family muridae (gray,1821) in the Republic of Moldova. The wood mouse (a. sylvaticus) is an eurytopic species and one of the dominant rodent species in the ecosystems of the Republic of Moldova. The studies were performed during 2010-2014 in the central part of the Republic of Moldova in various types of ecosystems: agricultural, fallow ground, meadows, pastures, woods, forest edge and shelter belts. The animals were caught with snap traps and live traps placed in lines at 5 m distance between the snap traps and at 10 meters between live traps during 2-4 days consecutively in springautumn period. In caught animals the following parameters were registered: age, sex, reproductive and physiolgical status. The ecological analysis of the collected data revealed the following peculiarities of the studied species: it was dominant in the majority of the investigated biotopes, with the abundance of about 48%-56% in agricultural ecosystems and at their limits, especially when shelter belts are in proximity, up to 70% in fallow grounds and abandoned lands, of 19%-44% at forest edge, of 16%-53% in shelter belts, depending on their location and composition. The species had a lower abundance (2%-6%) in forest ecosystems and was mostly recorded in the marginal areas of the woods. It doesn’t occur in deep and compact areas of the forest, it can be met in forest glades, cuttings and clearings. After the ecological significance the species is constant in fallow ground and abandoned lands, is constant and characteristic in agricultural ecosystems, at forest edge and in shelter belts and is accessory or accidental in forest biotopes. Therefore, the wood mouse, with high species number in the majority of studied biotopes, being host species for many parasite infections, was chosen for parasitological studies, in order to reveal the diversity of its helminth fauna. The parasitological assessment was performed by determining the prevalence (%), intensity (specimens), abundance (specimens) and the total invasive index (%) of parasitic species in a. sylvaticus. The indexes of prevalence, intensity and abundance were as follows in the following parasitic species: plaghiorchis elegans – 10.7%, 19.0%, and 0.67; hydatigera taeniaformis – 10.7%, 2, and 0.2; skrjabinotaenia lobata – 7.1%, 3 and 0.2; taenia pisiformis – 3.57%, 1, and 0.03; syphacia stroma – 10.7%, 16, and 1.7; syphacia obvelata – 7.1%, 90, and 6.4;  heligmosoma polygirus – 3.57%, 9, and 0.32; strongyloides ratti – 3.57%, 10, and 0.35;  trichocephalus muris – 14.2%, 2, and 0.14. Thus, the prevalence of Trematoda species was 3.6%, its intensity – 19.0, and its abundance – 0.67%, of the Cestoda species was 21.4%, 2, and 0.43, and of the Nematoda species – 35.7%, 22.4, and 8.8. The total invasive index was 46.4%, the total intensity index – 19.8, and the total abundance index – 18.8. The biological structure of the helminth fauna is formed by three groups of parasites with different types of biological cycle. Biohelmints, in which evolutionary cycle require the presence of several hosts, are represented by 4 species, of which one species from class Trematoda (plagiorchis elegans) and 3 species from class Cestoda (skrjabinotaenia lobata, taenia pisiformis larvae, hydatigera taeniaformis larvae). Geohelminths do not require intermediate hosts, but the larvae live free and feed in open environment with successive development. This category of parasites includes 3 species from class nematoda (trichocephalus muris, heligmosomoides  polygirus, strongyloides ratti). Ageohelminths comprise 2 species from class nematoda (syphacia stroma, syphacia obvelata), in which the females leg fertilized eggs in the perianal region of the host, while the infection occur through autoinvasion or individual contact between the hosts, thus omitting the development in open environment. Therefore, it was emphasized the helminth fauna in studied species is formed by 9 parasite species, of which 4 biohelmint species (44.4%), 3 geohelmint species (33.3%) and 3 ageohelmint species (22.2%). The study was performed under the projects 11.817.08.14F and 11.817.08.16A financed by the Academy of Sciences of Moldova.