Male and female Himalayantahr Hemitragus jemlahicus exhibit extreme sexual size dimorphism andare spatially segregated outside the winter mating season. Segregation results ina greater change in both habitat selection and diet of males relative to females.Since the structure of the ruminant digestive tract is closely related to feedinghabit, the rumen morphology of male and female tahr should reflect theseapparently important seasonal changes. We therefore hypothesized that the rumenmorphology of male tahr would undergo greater change between winter (when thesexes are aggregated) and summer (when the sexes are segregated) compared tofemales. Samples of rumen wall mucosa were cut from the dorsal rumen wall, atriumruminis, caudo-ventral blindsac, and the ventral rumen wall of tahr collectedduring winter (eight male, 10 female) and summer (11 male, 10 female) from theTwo Thumb Range, South Island, New Zealand. Although we recorded a significantseasonal increase in the mean surface enlargement factor (SEF) from winter tosummer for both sexes, the SEF of females and males were not significantlydifferent in winter (3.88 ± 0.55, SE, and 4.95 ± 0.61,respectively) or summer (7.71 ± 0.55 and 6.86 ± 0.52). We concludethat the effects of inter-sexual differences in habitat selection and diet onrumen morphology were insignificant relative to the extreme seasonal changes inforage quality and quantity that occur in the Two Thumb Range.