To protect the biosecurity of research rodent colonies, research institutions frequently require a quarantine period for live animals transferred into their facilities. Quarantine practices often include antibiotic and antiparasitic treatment with drugs such as fenbendazole and macrolide lactones. The influence of these compounds on the resident gut microbiota of mice is unknown, and any effects might subsequently affect model reproducibility. To test the influence of standard quarantine procedures on the composition of the microbiota, C57BL/6 mice, purchased from 2 different commercial suppliers, were randomly assigned to treatment groups (n = 12) by vendor and treated with fenbendazole-supplemented feed, topical moxidectin, both treatments, or no treatment (control), according to our institution's standard treatment regimen and duration. Feces were collected on arrival, immediately after completing the 8-wk treatment, and at 2 and 4 wk after treatment. Fecal DNA was extracted, sequenced, and analyzed to compare the changes in the microbiota of treated and control groups. Although significant main effects of time and treatment and interactions between those variables were detected in comparisons of richness, α-diversity, and β-diversity, the effect sizes associated with any particular treatment were consistently much smaller than that associated with acclimation to a new facility in the absence of any quarantine treatments. This outcome, along with the visual evaluation of principal coordinate analysis based on multiple similarity indices, suggests that time or institution plays a larger role in alterations of the murine gut microbiota than do quarantine treatments on its composition.