Differentiating between Fusarium strains is very difficult to realize because of its diversity in species and formae specials. In order to identify a strain of Fusarium isolated from leaves of tomato hybrid (Chifa) and evaluate its virulence towards this crop, it has been inoculated in tomato hybrid (Tafna). Morphological and microscopic aspects of Fusarium colony have been defined then inoculated, through Koch’s postulates, in seedlings and fruits using a spore suspension (1.5× 105 spores/ml). Treatment of plants has revealed that the isolate belongs to Fusarium oxysporum f. sp. lycopersici W.C. Snyder & H.N. Hansen, (1940) with high disease incidence (100 %) and complete infection obtained on tissues (roots, leaves, petioles and fruits). Statistical analysis (ANOVA) has confirmed the significant potential (P ≤ 5%) of this strain in appearance of obvious typical symptoms of disease on seedlings while the control subjects have been remained intact. Morphological and microscopic characteristics as well as inoculation in plant host are main tools to identify forma specialis of Fusarium if the host plant is already defined.