The objective of this work was to evaluate in vitro and in vivo the bioactivity of the essential oil of Cymbopogon citratus on the mycelial growth and sporulation of Alternaria solani, and also some biochemical and structural mechanisms of resistance induction on tomato plants. The plants were treated with essential oils at 0, 250, 500, 750, 1000 and 1500 μL L-1 72 hrs before the fungi inoculation. Foliar discs were collected at 0, 12, 24 and 48 h after the inoculation to verify the activity of peroxidase, polyphenol oxidase and β-1,3 glucanase. Spore germination, appressorium formation, and the mechanisms of structural resistance were evaluated 48 h after the inoculation. The in vitro responses showed direct toxic activities unlike the ones observed in vivo, where the essential oils had no fungitoxic effects on the fungi spores. In vitro, the peroxidase and polyphenol oxidase enhanced both local and systemic activities, unlike the β-1,3-glucanase which enhanced only local activities at the highest concentrations. The essential oils of C. citratus have, therefore, potential to induce resistance in tomato plants.