Biodegradation of 2,4,6-tribromophenol (TBP) was investigated in lowpermeability fractured-chalk cores. Long-term (-600 days) biodegradation experiments were conducted in two cores (-21 cm diameter, 31 and 44 cm long, respectively) intersected by a natural fracture. The impact of residence time, oxygen concentration, and chalk characteristics (such as pore size) on biodegradation were evaluated. In addition, the relationship between microbial activity and fracture transmissivity was evaluated. The main limiting factor for TBP biodegradation in these experiments was oxygen availability. Although the matrix pore-size distribution limits microbial activity to the fracture void, which has a relatively low surface area with respect to that of the entire chalk matrix, the chalk appears to provide an excellent environment for biodegradation activity. TBP removal was very slow when the conditions were similar to those expected in contaminated aquitards (natural attenuation). A significant enhancement in TBP removal was achieved by an increase in oxygen concentration within the fracture and faster flow rates, simulating an in situ bioremediation scenario, but at the same time, the fracture's transmissivity was reduced due to bioclogging. Approximately 90% of the TBP removal occurred within 10 cm of the TBP source, even when the residence time was reduced from 305 to 8 minutes and the fracture transmissivity decreased by up to two orders of magnitude (indicating that most of the biodegradation and clogging occurred near the contaminant source). The results obtained from this study suggest that in situ bioremediation can be used to accelerate the removal of organic contaminants in low-permeability fractured rock, if nutrient-delivery pathways within the aquitard are secured.