Aggressiveness of 10 Fusarium verticilloides isolates originated from naturally infected maize grains were studied towards susceptible cultivar ,Three Way Cross(TWC-310) seedling. Isolates differentiation has been done based on pathogenicity parameters as: a percentage of non-emerged seedlings, survival plants, plant vigour and disease severity. All tested isolates expressed various degrees of pathogenic capabilities as slightly, moderately and highly virulent. Percentage of survival plants and vigour were reduced, while disease severity ratio increased from 2.7 - 51.6%. Fumonisin production can play a role not only in plant pathogenicity but also in pathogen aggressiveness. The study suggested to correlate between the ability of examined isolates to produce that mycotoxin in vitro with their phytotoxicity behaviour. Isolates have been screened if some or all can synthesize fumonisin B1, whereas relatively different levels were detectable ranging from (400-1.680 µg/ mL). So, positive relationship between the aggressiveness of tested Fusarium isolates and their ability to produce this metabolite in vitro.