Several studies have shown that ecosystem functioning increases with increasing species richness. Most of these studies examined the effects of species richness on primary productivity. The underlying mechanism that explains this pattern is usually the selection effect. The higher the diversity in plant communities the higher the chance in including a very productive species that dominates the community, or a legume species that brings N into the soil. Less attention has been given so far to the effects of species richness on phosphorus exploitation. The aim of this work was to investigate the effect of species richness on aboveground primary productivity and P accumulation in a plant diversity experiment. For this reason, 14 grassland plant species were grown in containers as monocultures and in mixtures of 2-, 3-, 4-, 8-, 11- and 14-species combinations. Results show that the aboveground phytomass and total P increased with increasing species richness. Complementarity effects, probably through partitioning of resources, were most apparent in the highest levels of species richness, and were observed to be greater for total P in comparison to phytomass. Selection effects generally were greater for phytomass than for total P; they were significantly positive at the 2- to 8-species combinations but close to 0 or negative in the highest levels of species richness. The increases in phytomass and total P at the highest levels of species richness appeared to be caused by the increased performance of intermediate-productive species.