The objective of the present investigation consisted in evaluating in vitro the inhibitory effectiveness of the essential oil (AE) of leaves of Cymbopogon citratus in the Fusarium oxysporum present in tomato (Lycopersicum esculentum). The work was carried out in the Research Laboratory in Environmental Chemistry located in the University Corporation of Huila-CORHUILA, in the municipality of Neiva (Huila). It began with the extraction of AE from leaves of C. citratus, using the method of hydrodistillation with steam drag, after the extraction the sample of AE was sent to the laboratory of the UIS, where it was characterized by gas chromatography coupled to mass spectrometry, then the performance of the AE was calculated, obtaining 0,5 %.Subsequently, the leaf samples were collected, fruit and stem with contamination traits were collected to be incubated for seven days in Petri dishes with culture medium, until obtaining a strain of pure fungus, isolated the strain, were compared in a microscopic way by applying the atlas Mycology of Saldarriaga and some taxonomic keys, managing to identify F. oxysporum.The fungicidal activity was analyzed by direct seeding on the surface of potato dextrose agar in concentrations of 1000, 1500 and 2000 ppm with five repetitions, besides using two samples of absolute control and two samples of commercial control, during seven days. The results were favorable since the three concentrations observed a fungicidal percentage of 100 %, this may be due to citral (mixture between two stereoisomers, genarial 48.2 % and neral 34.9 %). Reason why, it can be assured that the AE of C. citratus could be a viable solution to the problems of this phytopathogenic fungi.