Anaplasma marginaleis a tick-borne pathogen that causes bovine anaplasmosis, which affects cattle around the world. Despite its broad prevalence and severe economic impacts, limited treatments exist for this disease. Our lab previously reported that a high proportion ofRickettsia bellii, a tick endosymbiont, in the microbiome of a population ofDermacentor andersoniticks negatively impacts the ticks’ ability to acquireA. marginale. To better understand this correlation, we used mixed infection ofA. marginaleandR. belliiinD. andersonicell culture. We assessed the impacts of different amounts ofR. belliiin coinfections, as well as establishedR. belliiinfection, on the ability ofA. marginaleto establish an infection and grow inD. andersonicells. From these experiments, we conclude thatA. marginaleis less able to establish an infection in the presence ofR. belliiand that an establishedR. belliiinfection inhibitsA. marginalereplication. This interaction highlights the importance of the microbiome in preventing tick vector competence and may lead to the development of a biological or mechanistic control forA. marginaletransmission by the tick.