traumatic brain injury (tBi) is often accompanied by gastrointestinal and metabolic disruptions. these systemic manifestations suggest possible involvement of the gut microbiota in head injury outcomes. Although gut dysbiosis after single, severe tBi has been documented, the majority of head injuries are mild, such as those that occur in athletes and military personnel exposed to repetitive head impacts. therefore, it is important to determine if repetitive, mild tBi (rmtBi) will also disrupt the gut microbiota. Male mice were exposed to mild head impacts daily for 20 days and assessed for cognitive behavior, neuropathology and disruptions in the gut microbiota at 0, 45 or 90 days after injury. Deficits in recognition memory were evident at the late post-injury points. Brains show an early increase in microglial activation at the 0-day time point that persisted until 90 days post-injury. This was compounded by substantial increases in astrocyte reactivity and phosphorylated tau at the 90day time point. in contrast, changes in the microbial community were minor and transient, and very few differences were observed in mice exposed to rmTBI compared to controls. While the progressive emergence of white matter damage and cognitive alterations after rmtBi resembles the alterations observed in athletes and military personnel exposed to rmtBi, these changes could not be linked to systematic modifications in the gut microbiota. Traumatic brain injury (TBI) results from a blow to the head and its severity can range along a continuum from mild (e.g., brief change in mental status or consciousness) to severe (e.g., extended unconsciousness, coma, prolonged amnesia) to fatal. TBI is one of the most common neurological diagnoses in the US 1 , with a CDC estimate of approximately 2.5 million people on an annual basis. About one-third of all injury deaths arise from TBI and roughly 90,000 persons experience long-term disability yearly as a result of brain injury 2. The cost of lifetime care for survivors of TBI has been estimated at $0.6-$1 million per individual with a total annual cost to the United States of ~$60 billion 3 , reflecting the severity of the dysfunction that results from this injury. Perhaps the form of TBI that has garnered the greatest scrutiny recently, in the public eye as well as within military, scientific and medical communities, is repetitive, mild TBI (rmTBI). Military operations in Iraq and Afghanistan are revealing that TBI accounts for about 28% of all combat casualties 4 approximately 85% of which are mild 4-6. More than 2.4 million US military personnel have served on multiple combat tours, and the US Defense and Veterans Brain Injury Center has estimated that ~413,000 military service members have been diagnosed with mTBI from 2001-2011 (https://dvbic.dcoe.mil/dod-worldwide-numbers-tbi). Besides the military,