The Asiatic ibex is a threatened endemic species that is distributed in the mountains of central and northern Asia. Using the method of group scan sampling, the behaviour of the Asiatic ibex was studied in the autumn of 2005. The effects of group size and sex on vigilance were tested. The results indicated that both group size and sex affected vigilance levels. Male ibex were significantly more vigilant than females at both the group scan level (percentage of individuals scanned during a session) and group scan frequency (percentage of intervals with at least one individual scanning). The group scan level was negatively correlated with group size, and group scan frequency was positively correlated with the group size in male, female, and overall groups. These results showed that group size and sex affected vigilance in Asiatic ibex.