Technical Abstract: Hexazinone, a triazine herbicide that is often detected as a ground and surface water contaminant, inhibits electron transport in photosynthetic organisms and is toxic to primary producers that serve as the base of the food chain. This laboratory study evaluated the ability of two types of microbial reactors, i.e., a vegetable-oil based nitrogen-limiting biobarrier and an aerobic slow sand filter, as methods for removing hexazinone from simulated groundwater. N-limiting biobarriers degraded hexazinone, but did so with a 52 week incubation period and a removal efficiency that varied greatly between replicates, with one biobarrier showing a removal efficiency of ~ 95% and the other an efficiency of ~ 50%. More consistent degradation was obtained with the aerobic sand biobarriers. Four aerobic biobarriers were evaluated and all behaved in a similar manner degrading hexazinone with removal efficiencies of ~97%; challenging two of the aerobic biobarriers with large amounts of influent hexazinone showed that these barriers are capable of remediating large amounts, > 100 mg L-1, of hexazinone at high efficiency. The remediation process was due to biological degradation rather than an abiotic processes. The long lag phase observed in both types of reactors suggests that an acclimation process, where microorganisms capable of degrading hexazinone increased in numbers, was required. Also, the isolation of bacteria that show a positive growth response to the presence of hexazinone in their growth media suggests biological degradation.