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This study involves three species of the family Orobanchaceae: Cistanche phelypaea, C. tubulosa and Orobanche cernua. These species are obligate perennial root parasites. They may occur together infecting hosts growing in the following phytogeographical regions: (1) along the Mediterranean coastal belt in deep oolitic limestone sand dunes and in loamy salt affected ground; (2) southern Sinai; and (3) south-eastern Egypt in Gebel Elba. In the Galalah and the Arabian desert regions, the parasite-host associations grow in silt terraces and runnels of the main wadis. The three root parasites were recorded to infect 16 different host species belonging to eight families from the Dicotyledoneae. Patterns of host specialisation occur in both Cistanche species where about 50% of their hosts belong to the Chenopodiaceae, while O. cernua exhibits a wider host range since it infects desert perennials and some cultivated crop plants. Compared to non-infected plants, it is apparent that Hammada elegans and Anabasis articulata Meg., which are hosts of C. phelypaea and C. tubulosa, respectively, were particularly sensitive to infection. The reduction in fresh and dry mass of such hosts was much higher than found in other hosts of Cistanche or O. cernua. The water content and succulence ratios of the parasitic species were significantly higher than their hosts and comparable non-infected species. The specificity of the parasites towards succulent hosts was higher in both robust species of Cistanche than in the less robust species O. cernua. (C) 1996 Academic Press Limited