Alopecia in Rickett's big-footed bat Myotis ricketti (Chiroptera: Vespertilionidae) in relation to age and sex. Zoological Studies 51(4): 494-499. We report alopecic syndrome (hair loss) in Rickett's big-footed bat Myotis ricketti (Chiroptera: Vespertilionidae), in Guilin, China, from June 2009 to Sept. 2010. The coat condition of bats was scored on a 3-point scale: AS0 (normal), AS5 (sparse hair), and AS10 (alopecia). Abnormal hair patterns were observed between early June and late Sept. when fresh hair again covered the abnormal region. The prevalence of alopecic syndrome varied in relation to age and sex, and reached a peak in late June. Adult females showed significantly higher incidences of prevalence of alopecic syndrome (90.9%) than adult males (55.3%) (χ^2 = 23.0, d.f. = 1, p < 0.01) from June to Aug. However, in immature bats, alopecia patterns were not observed in either sex, and only a sparse (AS5) hair condition occurred. No difference in the incidence of the condition was noted between males (51.5%) and females (60.0%) (χ^2 = 0.8, d.f. = 1, p > 0.01) from July to Aug. There was a significant difference between adults and juveniles (age) in the scale of occurrence of alopecia, and also its prevalence (χ^2 = 11.4, d.f. = 1, p < 0.01). We propose that parasitism, androgens, anthropogenic activities, or a combination of these factors might account for age and sexual differences in alopecia.