Characterizing the ways in which environmental microorganisms interact with one another has proven instrumental in allowing us to understand important ecological relationships in both natural and agricultural environments. To this end, diverse bacterial lineages have been shown to interact intimately with fungi, invading their hyphae to form an endosymbiosis. However, not much is currently known about the mechanisms enabling such interactions. A species of filamentous endophytic fungus, an ascomycete belonging to the genus Pestalotiopsis, was found to harbor the facultatively endosymbiotic bacterium Luteibacter mycovicinis. Both organisms are culturable in isolation. We have shown that axenic bacterial and fungal cultures can be reassociated together in-vitro, and hypothesize that flagellar motility might play a critical role in reassociation. To investigate this possibility, we created a mutant strain with a clean deletion in two genes encoding essential components of the flagella of endosymbiotic Luteibacter, which exhibits a non-motile phenotype. An assay developed for quantifying bacterial-fungal reassociation was then performed using these mutant strains. We report that mutant strains with impairments in flagellar motility show significantly less reassociation with Pestalotiopsis than wild-type Luteibacter. Furthermore, complementation of these genes recovered wild-type reassociation. This suggests that flagellar motility is required for successful establishment of endosymbiosis. Further investigation of the nature of flagellar genes and their regulation in endosymbiotic Luteibacter strains may allow us to increase our understanding of the entry mechanism for facultative endohyphal symbionts such as these, as well as the impact that these types of interactions may have on the larger ecosystem they inhabit.