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<i>Anaplasma marginale</i> is 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 of <i>Rickettsia bellii</i>, a tick endosymbiont, in the microbiome of a population of <i>Dermacentor andersoni</i> ticks negatively impacts the ticks’ ability to acquire <i>A. marginale</i>. To better understand this correlation, we used mixed infection of <i>A. marginale</i> and <i>R. bellii</i> in <i>D. andersoni</i> cell culture. We assessed the impacts of different amounts of <i>R. bellii</i> in coinfections, as well as established <i>R. bellii</i> infection, on the ability of <i>A. marginale</i> to establish an infection and grow in <i>D. andersoni</i> cells. From these experiments, we conclude that <i>A. marginale</i> is less able to establish an infection in the presence of <i>R. bellii</i> and that an established <i>R. bellii</i> infection inhibits <i>A. marginale</i> replication. 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 for <i>A. marginale</i> transmission by the tick.