Abstract: Blue sheep (Pseudois nayaur) are the principal prey of the endangered snow leopard (Panthera uncia) in the Himalayas and adjacent ranges. We studied group size and population composition of blue sheep in Manang District, Annapurna Conservation Area, Nepal. Overall mean group size was 15.6 (SE = 1.3), but it varied seasonally (P lt 0.001), with significantly smaller groups in winter than in other seasons. Mixed groups were most numerous in all seasons, and there was no evidence of sexual segregation. Yearling sex ratio (93.7 M:100 F) did not vary seasonally, nor did the ratio deviate from parity. Adult sex ratio showed a seasonal pattern favoring males post-parturition but female-biased during the rut and pre-parturition. Seasonal variation in sex-specific mortality is offered as a plausible explanation for the observed pattern in adult sex ratio.