Sex ratios of populations of the dioecious shrub Pistacia lentiscus L. (Anacardiaceae) were studied. Several hypotheses concerning biased sex ratios were tested. The expected pattern of male preponderance in stressful habitats was not found. The populations located in a microclimatic gradient, such as a slope, did not display a male-biased sex ratio on the stressful middle slope. The populations located in a climatic gradient did not display a male-biased sex ratio in the more xeric habitats. Testing the hypothesis of female preponderance when pollen grain competition exists, we found a significant correlation in the direction opposite to that predicted by this hypothesis. Low density of individuals (an estimate of pollen density) correlates with a high preponderance of females but the sex ratio approaches 1:1 when density increases. This correlation should have an upper threshold in 1:1 because male-biased sex ratios have never been found.