We studied from 1993 to 2002 the spatial organization of visually marked females in an isard ( Rupicapra pyrenaica Bonaparte, 1845) population in the Pyrenees National Park. In winter, all individuals were concentrated within the same 3.7 km² refuge area. In summer, two patterns of space use were recorded: sedentary females (n = 33; 52.2%) remained within the refuge area, whereas migrating females (n = 29; 46.8%) summered outside the refuge area, at a horizontal distance of 1.4–9.2 km and 300–800 m higher in elevation than their winter ranges. Females were faithful to their seasonal ranges. The population was organized in well-defined spatial units in summer. This organization was roughly preserved in winter, despite the spatial concentration of the individuals by this season. The resulting spatial clustering might reduce spatial competition and pathogenic contamination between groups, allowing the maintenance of high population density. The appearance of a migratory pattern occurred probably as a result of the protected status of the species, which allowed the population to increase. Some individuals dispersed as a result of the population density increase. The two recorded patterns of space use likely arose from spatial attachment and social affinity that developed in the first years of a female’s life, and could explain the coexistence of the two spatial patterns.