The spatial organization of capercaillie (Tetrao urogallus) cocks on a lek was studied in the French Central Pyrenees from 1979 to 1983 by mapping territory boundaries. Between 1980 and 1982 more detailed data were obtained on the movements of 14 cocks that could be individually identified by their plumage. The objectives were to determine if the boundaries of the lek varied from year to year and to study changes in size and location of areas defended by individual males on the lek, both within and between years. The spatial boundaries of the lek showed a certain plasticity from year to year owing to population turnover. Within the lek each individual male defended a restricted area corresponding to the definition of a territory. Each year I observed a spatial reorganization of these territories. Thus the capercaillie lek system is characterized by a certain individual plasticity which provides an explanation for the ability of this species to cope with different environments even though it cannot resist brutal modifications of large areas of its habitat.