—the Gyrfalcon is a circumpolar, arctic and resident raptor in Fennoscandia and is totally dependent on only one or two ptarmigan species, the Willow Ptarmigan (Lagopus lagopus) and rock Ptarmigan (Lagopus muta), for food over half of the year. the diet of the Gyrfalcon has been studied in many parts of its range in earlier decades, but recently only in a few regions. there is need for current knowledge of the availability and usage of prey, and regional and temporal variation of the diet, because the populations of the two Lagopus species seem to have declined in some parts of the Gyrfalcon s range. Availability of food may thus become a limiting factor for falcon populations, and it has been identified as the main threat for viability of falcon populations in the European range, together with the availability of secure nest sites. the published studies of inland populations come from alpine and tundra regions where the rock Ptarmigan is the most important prey species. In our study area in Finnish Lapland and neighbouring regions in northern Norway and sweden, during breeding seasons (1998 to 2010), Willow Ptarmigan was the main prey (73.6% of total Lagopus spp. by number). the two ptarmigan species formed 89% of the prey individuals, and this percentage varied little among regions and landscapes, or annually, despite strong cyclical fluctuations of the ptarmigan populations. the falcons feed their young almost exclusively on Lagopus spp. Other species of appropriate size (waders, waterfowl, etc.) were available only from late spring to late summer, and were hunted by falcons in small numbers. Received 11 March 2011, accepted 9 September 2011. KOsKImIEs, P., ANd s. suLKAvA. 2011. diet of the Gyrfalcon (Falco rusticolus) in northern Fennoscandia. In r. t. Watson, t. J. cade, m. Fuller, G. Hunt, and E. Potapov (Eds.). Gyrfalcons and Ptarmigan in a changing World. the Peregrine Fund, boise, Idaho, usA. http:// dx.doi.org/10.4080/gpcw.2011.0400