Dioecious plant species are an important component of terrestrial ecosystems. However, little is known about their ecology, including the factors affecting their distribution. We investigated the ecology of Populus cathayana Rehd, a dioecious and deciduous species that occupies a wide range of habitats in China. We measured the mean diameter at breast height (DBH), population density, sex ratio, size structure, and spatial distribution of male and female P. cathayana populations in a series of forest plots along an altitudinal gradient (1400-1700 m) in the Xiaowutai Mountains. The results showed that the trends in mean DBH and density were significantly different for males and females. There was no significant difference in mean DBH along the altitudinal gradient in female populations and the population density at 1700 m was lower than at other altitudes. Compared with females, the maximum mean DBH for males was observed at 1700 m and was significantly larger than at other altitudes. However, there was no variation in the population density of males associated with altitude. The mean DBH in male and female populations was nearly equal at 1600 m and the density at this altitude was also very similar among male and female populations. The sex ratio (male∶female) tended to increase along the altitudinal gradient. Although the sex ratio was close to 1∶1 (χ ² = 2.94, P > 0.05) for the entire population (1400-1700 m), the ratio was biased towards females (χ ²₁₄₀₀ = 5.91, P < 0.05) at low altitudes and biased toward males (χ ²₁₇₀₀ = 6.56, P < 0.05) at high altitudes. The sex ratio approached 1∶1 (χ ²₁₆₀₀ = 0, P > 0.05) as the altitude tended towards 1600 m. Furthermore, the size structure of all male and female P. cathayana populations revealed a steady-state distribution along the altitudinal gradient (except for a decline-state distribution at 1700 m). However, the distribution of the size classes differed between male and female P. cathayana populations. A large proportion of class Ⅰ and Ⅱ were observed at 1400 m in female populations but at 1600 m in male populations. In addition, we also found that the distribution of male and female populations was affected by altitude. We observed a random distribution pattern at low altitudes and a clumped distribution pattern in most male and female populations at other altitudes, though we did not detect a difference among the sexes. However, the aggregation intensity between male and female P. cathayana populations differed markedly along the elevation gradient. The aggregation intensity of female populations increased gradually along the elevation gradient whereas the change in aggregation intensity was nonlinear in male populations. Below 1500 m, the aggregation intensity increased with increasing altitude, but then decreased with increasing altitude above 1500 m. This phenomenon may be explained by differences in the adaptability of male and female P. cathayana plants. In summary, we showed that differences in the distribution of male and female P. cathayana populations along an altitudinal gradient. And we conclude that P. cathayana populations are likely to reproduce and survive best at moderate altitudes.