The reproductive ecology of the dioecious herb Aciphylla simplicifolia (F.Muell.) Benth. (Apiaceae, Mountain Aciphylla) was examined in Kosciuszko National Park. Differences in floral display and flowering phenology between male and female plants were consistent with predictions based on theories concerning sexual dimorphism in dioecious plants. For example, male plants had larger floral displays than females at the alpine sites sampled. Male inflorescences had four times as many flowers as females and more than three times the area of floral display. In addition to differences in floral display, there was a sex-specific pattern in flowering phenology at six alpine sites. At these sites, there were more male inflorescences with buds and flowers and more female inflorescences finished flowering than would be expected if gender did not affect flowering phenology. To determine whether increasingly severe conditions associated with higher-altitude sites were associated with male-biased sex ratios, the number of male and female plants were compared for 20 sites over a 600-m altitudinal range from montane to alpine (total 4274 plants). As altitude increased and as the vegetation zone changed from subalpine to alpine, the sex ratio became increasingly male-biased, with sites in the alpine ranging from 1.45 males per female to 8.53 males per female. Sexual dimorphism in floral display, flowering phenology and sex ratios was consistent with what would be predicted for plants with sex-specific differences in resource allocation.