Fusarium graminearum with the 15-acetyl-deoxynivalenol (15-ADON) chemotype is the predominant cause of Fusarium head blight (FHB) of wheat in the U.S. and Brazil. In the U.S., the same pathogen population causes Gibberella ear rot (GER) and stalk rot (GSR) of maize, and infected maize serves as the major source of inoculum for wheat. In contrast, in Brazil F. meridionale with the nivalenol (NIV) chemotype is the main cause of GER and GSR. We hypothesized that this shift in dominance on maize is explained by greater fitness of F. meridionale (which doesn’t occur in the U.S. on that host). A collection of Brazilian isolates of F. graminearum (10 from wheat and 10 from maize) and F. meridionale (10 from wheat and 10 from maize) were compared for several traits that are potentially relevant to fitness including aggressiveness to maize ears and stalks and to wheat heads; mycelial growth rate; macroconidial, perithecial, and ascospore production; and fungicide sensitivity. The two species could not be differentiated for aggressiveness to maize: there was more variation within species than between species for this trait. A few traits were more closely associated with species: most strains of F. meridionale were less aggressive on wheat, and produced fewer macroconidia, perithecia, and ascospores in culture, than most strains of F. graminearum. The goal of our studies is to improve management of FHB, GER, and GSR in Brazil, accounting for differences from U.S. conditions.