The European beech (Fagus sylvatica L.) is at present considered to be the mostcommon economically important and wiedspread tree species in Europe. In contrast to othertree species and economically important species in Croatia European beech has the largestareal and can be found in variety of plant communities. Beech forests represent biotope fordifferent plant and animal species including small mammals (rodents). As the most speciousand widely distributed mammalian group, rodents are the wildlife reservoir for manyzoonoses which can infect humans and other wildlife. Pathogens can be transmittedhorizontally via direct or indirect contacts to host species and humans. Rodent outbreaks tendto follow years with increased seed production of oak and beech. Many authors have linkedfluctuations in abundance of rodent reservoirs with increase of human zoonotic infections. Inforests the most. Aim of this research is monitoring of small mammal populations and theirzoonosis including hantaviruses and leptospira species, rodent species interactions, theirspatial distribution and habitat use. At two locations (National park Plitvice lakes and Naturepark Medvednica) seven trapping plots were set, each containing 1 or 2 grids size 100m x100m (100 traps). Rodent capturing was done from year 2011 until 2014 in summer andautumn. Trapped rodents were screened for hantaviruses and leptospira species. Their sex, ageand reproductive status was determined, body mass and morphometry was measured. Ongrids in National park Plitvice lakes microhabitat parameters were collected for each trapposition as well as yearly beech seed production. Results show two dominant rodent speciesyellow-necked mouse (Apodemus flavicollis) and bank vole (Myodes glareolus) in beechforests of National Park Plitvice and Nature Park Medvednica. Both species make 99 % oftotal captures. Remaining 1 % is made of two common shrews (Sorex araneus), one edibledormouse (Glis glis) and one vole from genus Microtus. Rodent abundance showed increasein year 2012 and outbreak in year 2014 measuring on one grid max abundance of 45animals/ha, whereas year 2011 and 2013 had lowest abundance. Beach mast was reported foreach year by counting the amount of beech seeds on the ground at each trap position on 1 m2surface. In 2011 there was slight increase in beech seed production reported in comparison toyear 2012 and 2014. In year 2013 beech seed production was the highest with mean beechseeds/m2 23x higher than in year 2012. From the rodent abundance and beech mast data wesee the trend of mast years being followed with high rodent abundance. Microhabitatparameters measured for each trapping grid showed that bank voles are positively associatedwith tree sapling cover. For yellow-necked mouse there was no association found withVIIImeasured microhabitat parameters. Spatial distribution of two dominant rodent species didntshow any difference. No correlation was found between yellow-necked mouse and bank voleoccurrence within grids. In beech forests of National Park Plitvice lakes and Nature ParkMedvednica 3 different hantaviruses were found within their typical rodent host species:Puumala (PUUV) in bank voles (Myodes glareolus), hantavirus Dobrava (DOBV) in yellowneckedmouse (Apodemus flavicollis) and Seewis (SWSV) in common shrew (Sorex araneus).Highest infection rate was found in bank voles with 50 % of Puumala positive individuals.Dobrava was found in 5 % of trapped yellow-necked mouse. From two trapped commonshrews one was positive to Seewis and makes first molecular evidence of shrew-borne Seewisvirus in Croatia. From two yellow-necked mouse males 2 different leptospira species wereisolated: Leptospira interrogans (serovar Australis) and Leptospira kirschneri (serovarPomona). For same two males dual infections were found with listed leptospira species andDobrava virus. Highest proportion of hantavirus positive rodents was found in year 2014when their abundance was also reported highest. Between both locations significantdifference was found in tail and hind foot length of bank voles, being longer in National ParkPlitvice lakes. Tail length of yellow-necked mouse was also found longer in National ParkPlitvice lakes compared to ones from Nature Park Medvednica which indicates possible localadaptations. From this data the connection of beech mast year and rodent abundancefollowing next year as well as high infection rates of rodents is confirmed. In same years withincreased rodent abundance there was high number of human HFRS cases (hemorrhagic feverwith renal failure syndrome) which confirms the influence of beech mast on rodent abundanceand increasing human cases.