Water relations of the mistletoe Amyema fitzgeraldii and its host Acacia acuminata were studied near Geraldton, Western Australia. Transpiration rates of host and parasite under unstressed winter conditions varied more than 300-fold between day and night while leaf water potential gradients between the partners remained within the range 0.4-0.6 MPa. Plots of transpiration against leaf water potential indicated closely similar fluid phase resistances in host and parasite during daytime but divergent behaviour at night due to an apparently large increase in resistance of the haustorial junction between the partners. Data for summer and winter studies across a full range of light intensities showed the parasite to transpire, on average, 1-4 times faster and to exhibit noticeably lower water use efficiencies than its host. Experiments following restorative changes at night in leaf water potentials of host and parasite on detached host branches supplied through their cut ends with water indicated that the haustorium offered a major resistance to water uptake by the parasite. Restoration of leaf water potentials by the parasite lagged markedly behind that of the host, especially during winter, leading to a rapid build up in potential gradient between partners. A phase of rapid flow into the parasite then followed, presumably motivated by lowering of the haustorial resistance. Reversal of the potential gradient between host and parasite was recorded in a night-time study involving a bagged (non-transpiring) mistletoe attached to a host branch frozen at the base to prevent further water uptake. Mechanisms are proposed to account for the apparently highly variable nature of the resistance of the haustorium.