The potential of phytoremediation as a treatment option for olive mill wastewater (OMW) was tested on five perennial tree species. Cupressus sempervirens and Quercus ilex proved tolerant to six-month OMW treatment followed by six-month water irrigation, whereas Salix sp. and Laurus nobilis and, later, Pinus mugo suffered from phytotoxic effects. Test plants were compared to controls after treatment and irrigation, by monitoring biochemical and microbiological variations in the rhizosphere soil. OMW-treated soils were exposed to 50-fold higher phenols concentrations, which, irrespective of whether the respective plants were OMW-resistant or susceptible, were reduced by more than 90% by the end of the irrigation cycle, owing to significantly increased laccase, peroxidase and beta-glucosidase activities, recovery/acquisition of bacterial culturability and transitory development of specialized fungal communities sharing the presence of Geotrichum candidum. Of all results, the identification of Penicillium chrysogenum and Penicillium aurantiogriseum as dominant rhizosphere fungi was distinctive of OMW-tolerant species. (C) 2010 Elsevier Ltd. All rights reserved.