Technical Abstract: Previous studies have demonstrated the presence of multiple strains of Marek’s disease virus simultaneously circulating within poultry flocks, leading to the assumption that individual birds are repeatedly exposed to a variety of virus strains in their lifetime. Virus competition within individual birds may be an important factor that influences the outcome of coinfection under field conditions, including the potential outcome of emergence or evolution of more virulent strains. A series of experiments were designed to evaluate virus competition within chickens following simultaneous challenge with two fully virulent S1 MDV strains, using either similar (rMd5 and rMd5//38CVI) or dissimilar (JM/102W and rMd5//38CVI) virus pairs. Bursa of Fabricius, feather follicle epithelium, spleen, and tumor samples were collected at multiple time points to determine the frequency and distribution of each virus present by using pyrosequencing, immunohistochemistry and virus isolation. In the similar pair, rMd5 appeared to have a competitive advantage over rMd5//38CVI, which in turn had a competitive advantage over the less virulent JM/102W in the dissimilar virus pair. Dominance of one strain over the other was not absolute for either virus pair, as the subordinate virus was rarely eliminated. Interestingly, competition between two viruses with either pair rarely ended in a draw. Further work is needed to identify factors that influence virus-specific dominance to better understand what characteristics favor emergence of one strain in chicken populations at the expense of other strains.