The aim of this study was to determine and compare the level of colonization of poplar roots with ectomycorrhizal (ECM), arbuscular mycorrhizal (AM) and dark septated endophytic fungi (END) and to study the effects of site conditions on these parameters. Four sites with poplars were selected: at the first location "Rasadnik" a white poplar clone multiplied in vitro was grown, the second site "Koviljski rit" was a natural habitat of native white poplar, the soil at the third site "Timok" was contaminated with pyrite tailings, contained heavy metals and had a low pH, while the fourth site "Antella" was a post- agricultural field in which ambient ozone impacts on an ozone-sensitive clone were investigated. The presence of AM fungi was not observed at the "Timok" site while a high number was recorded in "Antella" where it amounted 24.76%. The value of poplar root length colonization with AM fungi and AM / ECM ratio significantly differed between the two sites. On the other hand, poplar rooth length colonization with ECM fungi was even, ranging from 16.7% in "Koviljski rit" to 21.84% in "Antella". Dark septated endophytic fungi inhabited the poplar roots on analyzed habitats to a lesser extent and colonization of these species ranged from 1.38% in "Rasadnik" to 6.09% in the "Koviljski rit". No significant correlation between root colonization with AM, END and ECM fungi was found at any of the analyzed localities. Since colonization of roots with mycorrhizal and endophytic fungi was not a reliable parameter for mycobioindication, the total biomass and root length should be determined in further research, as well.