| Sumario: | (1) Sex ratio, plant size, spatial distribution and host-plant selection have been examined in three south-western Spanish populations of Osyris quadriparrira (Santala ceae), a hemiparasitic dioecious shrub of mediterranean habitats. The three populations are distributed along an environmental gradient characterized by variation in water availability and soil fertility. (2) Sex ratios do not depart significantly from 1: 1 in any of the populations studied. There is no significant spatial segregation of plants of the two sexes in any population, as assessed by nearest-neighbour analyses. (3) Significant dimorphism in plant size was found at one of the sites, where males were significantly larger than females. (4) The positive correlation between combined plant size and distance of separation of neighbours provides evidence for intraspecific competition in this species. The effect of intersexual competition is greater than the effect of intrasexual competition between males, but it is similar to the effect of intrasexual competition between females. Females apparently have a greater competitive effect on males than other males. (5) Males and females are similar in their proportional use of host-plant species at two sites, and differ significantly at the other locality. (6) These results provide no evidence for niche differentiation of the sexes of 0. quadriparrita along an environmental gradient defined by abiotic factors (moisture, soil nutrients), but favour the view of small-scale segregation based on differential utilization of host-plants. It is suggested that niche differentiation in parasitic dioecious plants is most likely to occur along biologically rather than physically defined environmental gradients
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