The role of the jasmonate signalling pathway in modulating the establishment of the arbuscular mycorrhiza (AM) symbiosis between tomato plants and Glomus intraradices fungus was studied. The consequences of AM formation due to the blockage of the jasmonate signalling pathway were studied in experiments with plant mutants impaired in JA perception. The tomato jai-1 mutant (jas- monic acid insensitive 1) failed to regulate colonization and was more susceptible to fungal infection, showing acceler- ated colonization. The frequency and the intensity of fungal colonization were greatly increased in the jai-1 insensitive mutant plants. In parallel, the systemic effects on mycorrh- ization due to the activation of the jasmonate signalling pathway by foliar application of MeJA were evaluated and histochemical and molecular parameters of mycorrhizal intensity and efficiency were measured. Histochemical deter- mination of fungal infectivity and fungal alkaline phosphatase activity reveal that the systemic application of MeJA was effective in reducing mycorrhization and mainly affected fungal phosphate metabolism and arbuscule formation, ana- lyzed by the expression of GiALP and the AM-specific gene LePT4, respectively. The results of the present study clearly show that JA participates in the susceptibility of tomato to infection by arbuscular mycorrhizal fungi, and it seems that arbuscular colonization in tomato is tightly controlled by the jasmonate signalling pathway.