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Olive trees have a great adaptability to adverse soil conditions and are typically grown in marginal soils with low fertility. Water deficit and osmotic imbalance are the most common stressors affecting crops in arid and semi-arid regions. Tolerance to osmotic stress in plants is a complex phenomenon that involves many changes at the biochemical and physiological levels. However, the mechanisms behind the modulation of tissue water conductivity and osmotic adjustment appear to be affected by arbuscular mycorrhizal fungi (AMF). This study aims to evaluate the effect of water availability on the AMF (colonization and spores) of olive trees ('Arbequina') grown in two different agro-climatic environments. The experimental design included two water deficit treatments, one at 50% of the estimated crop evapotranspiration in the province of Mendoza and a second one under natural rainfall conditions in the province of Cordoba, and in each locality two adjacent treatments irrigated at 100% of the estimated crop evapotranspiration. The root-associated soil samples from each individual plant (n=10) were randomly collected between 10 to 20 cm depth. Each sample consisted of five bulked sub-samples (200 cm(3) soil cores). From each soil sample, olive roots were extracted, stained and their AMF structures determined. The total AMF spore number was also quantified. The arbuscular mycorrhizal colonization (AMC) rate was higher in treatments with less water availability; the average increments were found to be 42 and 75% for Mendoza and Cordoba, respectively (p <= 0.01). The same pattern was found in AMF spore number; 42% for Mendoza and 121% for Cordoba (p <= 0.01). These results indicate that treatments with less irrigation seem to stimulate AMC and spore production. Further studies are needed in order to determine if AMF allows olive tree roots to access the soil water reservoir.