The study of diet composition of the Montagu`s Harrier (Circus pygargus (L) in two distinct breeding populations in 1996 in Hrodna and Smarhon` districts in Western Belarus have found different occurence of small mammals: 72,6% (n=2034) for the population #1 and 85,0% (n=254) for the population #2. Main prey in both populations was represented by vole species Microtus spp.: 68,5% and 78,3% for populations #1 and #2 accordingly. Significant difference was found in share of birds, mainly ground-nesting Passerines (17,5% and 12,6% for the population #1 and #2 accordingly) and large grasshoppers Tettigonia sp. (6,4% and 0,4% accordingly). Other prey categories (mice, shrews, hares, eggs of birds, lizards, dragonflies, large beetles and frogs) did not form an important part of the diet of these populations. We believe that opportunistic hunting behavior of the Montagu’s Harrier (which can easy switch on seasonally or locally abundant and accessible prey) is one of the reasons why the species did not form any subspecies throughout its wide range.