Measurements of groundwater head and hydraulic conductivity have been made around a fault in the Oxford Clay in order to examine the effect of a fault on the hydraulic properties of a clay and on groundwater flow. Pulse tests performed in the clays gave values of hydraulic conductivity ranging from 5 × 10-12 to 2 × 10-8 ms-1. Although showing some overlap with the unfaulted clays, the hydraulic conductivity in the fault zone appears to be enhanced with respect to the adjacent unfaulted clays by up to one or two orders of magnitude. The groundwater heads in the mudrocks/clays are high compared with those in the underlying aquifers and appear to be relics from the period before major groundwater abstraction. Some outline numerical modelling suggested that both changes in fault geometry and varying fault properties could be invoked to explain adequately the measured head profiles using the traditional assumptions of Darcian flow. However, it is recognized that coupled flow processes may be of great significance in clay formations; in particular, the measured heads may result from hydraulic effects coupled with chemical and mechanical potentials.