The parasitic Scrophulariaceae Bartsia, Euphrasia, Melampyrum, Odontites , and Pedicularis were studied in respect to host interactions. With laboratory experiments it is shown that the development of the root system of the parasit is not influenced by host roots in the early stages. Later, if hosts are absent, the root system becomes less voluminous. When host roots are present, normally the primary root of the parasit attacks the root first with a secundary haustorium. All side roots of the parasites, too, are able to produce secundary haustoria, but only by chance contacts when roots are close enough. Again, the phenomenon of selfparasitism in one and the same plant could be confirmed. The parasitic plant doesn’t show this phenomenon when normal host plants are absent. In the presence of another parasitic plant, parts of one and the same root can function as a host or as a parasite. In all cases of attachment, only one root of both develops a haustorium in the contact area. The most appropriate hosts are annual legumes. As under natural conditions the same result could be observed in the laboratories. It is remarkable that the bacterial nodules of the legumes are preferred for attachments. In field studies it could be shown that parasitic Scrophulariaceae do not attack orchids. Experiments under natural conditions in the field, and experiments in the laboratory, too, show that orchids are never attacked. Host plants, infected by vesicular arbuscular mycorrhiza fungi are not protected against these parasitic flowering plants. In fact, there seems to be a stimulation for the parasites to attack especially mycorhizal roots of the host. Also, the parasitic Scrophulariaceae can be infected easily by the VAM fungi. This kind of symbiosis probably helps the parasitic plant to grow without a host plant, but if the inoculation starts too early with seedlings, in the most cases the parasitic plants stop growing and die.