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Sexual segregation has been found among many vertebrate species. Argali, like other sexually dimorphic Capridae, form sexually segregated groups outside the breeding season. The degree of sexual segregation and its seasonal changes have never been examined and quantified in Asiatic species of wild sheep. In this paper we are considering seasonal fluctuation in the degree of sexual segregation and check the activity budget hypothesis to explain this phenomenon in the Darwin's wild sheep (Ovis ammon darwini), a subspecies of argali sheep (Ovis ammon). The activity budget hypothesis states that sexually size-dimorphic males and females segregate into different groups due to incompatibilities in activity budgets and movement rates. We collected data on activity budgets in the argali sheep from 2015 to 2016, in the Mengluoke Mountains of Xinjiang Province, and used Conradt's segregation coefficient (SC) to measure the degree of sexual segregation outside and during the rutting seasons. Our results showed that the SC value was highest outside the rutting season (0.98), when argalis were almost completely segregated. During the rutting period, the segregation coefficient dropped more than twofold (0.43) compared to the non-rutting season. In addition, our data supported the activity budget hypothesis: female and male argali had different behavioural activity patterns during the non-rutting season. Female argali spent more time feeding, followed by resting, standing, moving, and other behaviours, while males spent most of their time resting, followed by feeding, standing, other behaviours, and moving. Female argali spent significantly more time feeding than males, while males spent significantly more time resting and in other behaviours than females. Activity synchronization indices for both female groups and male groups were significantly higher than in mixed-sex groups. Sexual segregation is the best behavioural strategy for sexually dimorphic ungulates and the activity budget hypothesis explained this phenomenon well in Darwin's wild sheep. [ABSTRACT FROM AUTHOR]