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Xylem-tapping mistletoe species growing on Mimosaceae, non-Mimosaceae and hosts performing Crassulacean acid metabolism (CAM) were studied along an aridity gradient in the Namib desert. Delta-C-13-values of mistletoes became more negative with decreasing nitrogen (N)-concentration in their leaves, while the host plants showed no such relationship. This might suggest that mistletoes regulate their water use efficiency according to the nitrogen supply from the host. However, further inspection of the data indicates that the relations of delta-C-13-values with leaf nitrogen in mistletoes may result from carbon input from the host. This is especially true for mistletoes growing on CAM plants which exhibit a very high delta-C-13-value, but show no evidence of CAM. It is calculated that about 60% of the carbon in mistletoes growing on C3 and on CAM hosts originated from the host. The hypothesis of Marshall and Ehleringer (1990) that xylem tapping mistletoes are also carbon parasites could explain the change in delta-C-13-values with N-supply and the difference in delta-C-13-values between mistletoes growing on C3 and CAM hosts.