Fusarium graminearum, the most common causal agent of Fusarium Head Blight on wheat, can cause significant yield loss and release mycotoxins that are known to cause diseases in both humans and animals. The level of aggressiveness in F. graminearum is both heritable and a highly variable trait. Genetic resistance in wheat is an important part of FHB management and could affect the populations of F. graminearum. I hypothesize that the aggressiveness of F. graminearum is affected by the resistance level of the wheat source. Isolates of F. graminearum were collected from naturally infected wheat lines with different levels of resistance. For this research 31 isolates from highly susceptible and 26 isolates from moderately resistant wheat lines were used. We conducted in vitro phenotyping of these isolates by recording the area of mycelial growth at 2, 4, 6, 8, and 10 days and sporulation at 10 days. To assess the aggressiveness of the isolates, ‘Norm’ spring wheat was grown in the greenhouse and inoculated at anthesis. Three disease ratings were taken weekly after inoculations and used to calculate an AUDPC value. Once harvested the percentage of Fusarium diseased kernels (FDK) per head was calculated. The harvested kernels will then be used for quantification of the mycotoxin deoxynivalenol (DON). Both the in vitro phenotyping and aggressiveness assay experiments were repeated twice. DNA was extracted from all the F. graminearum isolates and sequenced on an SP lane of a Novaseq6000. A linear mixed model was used to analyze the isolate’s phenotypes and found that the isolate’s source significantly affects the AUDPC, in vitro growth rate, and spore count. Isolates from the susceptible wheat lines were more aggressive than isolates from the resistant wheat lines. A strong significant correlation was found between AUDPC and FDK. There was also a significant correlation between the in vitro growth rate and FDK showing isolates that grow faster in vitro produce more diseased kernels in planta. The whole-genome sequences are currently being analyzed. Results from the data analyzed so far show resistant wheat lines are not selecting for more aggressive isolates. From these results, a better understanding of the factors affecting individual strain aggressiveness can be achieved and be useful for managing the deployment of disease-resistant wheat varieties.