Abstract An overview is given about the δ(13)C- and δ5D-values in the organic material of hemi- and full- parasitic higher plants and of their hosts. Hemiparasites have to direct the content of the host xylem into their tissues by lowering their water potential. They achieve this by active water secretion or by lowering the stomatal resistance. In the latter case, the intercellular CO(2)-concentration in the chlorenchyme of the parasite is increased. This causes a reduction of the (13)C-content (δ(13)C-value of the parasite more negative than of the host). The dry matter of the mistletoes is always richer in deuterium than that of the host. The reason for this fact is unknown. In the case hemiparasites and their hosts show differences in the (13)CO(2)-discrimination during the photosynthetic CO(2)-fixation, an eventual transfer of organic material from host to parasite can be checked. By this way a holoparasitic mistletoe, Tristerix aphyllus, could be demonstrated. In contrast to mistletoes on C3-hosts, such on CAM-hosts have a lower deuterium content in the dry mass than the hosts. Holoparasites get all their organic material from the hosts and mirror, therefore, in their δ(13)C-values those of the hosts. Their deuterium content in the organic material is always higher than the one in the host. The reason is unknown.