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Acacia raddiana populations parasitized by the mistletoe Plicosepalus acaciae (Loranthaceae) are suffering high levels of mortality in many parts of the Negev Desert. It has been hypothesized that xylem-tapping mistletoes are profligate users of water in order to maintain a high rate of uptake of important nutrients (especially nitrogen) from their hosts. According to this passive nutrient uptake theory, mistletoes could cause water stress and even mortality in hosts. However, a number of studies have indicated that mistletoes may actively take up nutrients via the phloem. In this latter case, mistletoe nutrient uptake need not be related to profligate water use, with minimal effects on the water status of their hosts. We investigated the effects of P. acaciae on A. raddiana in the light of these theories. Mistletoe water potentials were always more negative than those of their hosts. The degree of mistletoe infestation did not affect Acacia water status or mortality. In contrast to the results of most other studies, mistletoes had lower nitrogen concentrations (65% less) than their hosts. Consistent with the passive nutrient uptake theory, there was a significant negative correlation between mistletoe water potentials and mistletoe nitrogen concentrations. The ratio of phloem-mobile nitrogen to phloem-immobile calcium in mistletoes was significantly greater than one, which is consistent with the active nutrient uptake theory. However, N:Ca ratio in mistletoes was 39% of that of their hosts. These results indicate that this mistletoe takes up nitrogen by both passive and active routes, although neither is very effective. We found that host quality had significant effects on the mistletoe: mistletoe canopies were significantly larger on high nitrogen hosts, and mistletoe flower production was negatively correlated with host water status. Our results indicate that hosts can limit or have a negative effect on parasitic mistletoes and do not support the hypothesis that the mistletoe contributes to host water stress and mortality. (C) 2003 Elsevier Ltd. All rights reserved.