© 2012 Inforesights Publishing UK 106 Phytopharmacology 2012, 2(1) 106-113 Introduction Natural products perform various functions, and many of them have interesting and useful biological activities (Harvey, 1999). There are more than 35,000 plant species being used in various human cultures around the world for medicinal purpose. Researchers are incr- easingly turning their attention to natural products looking for new leads to develop better drugs against cancer, as well as viral and microbial infections (Hoffmann et al ., 1993; Harvey, 1999; Srinivasan et al., 2001).More than 80% of the world's population relies on traditional medicine for their primary healthcare needs. Use of herbal medicine in Asia repre- sents a long history of human interactions with the environment. Plants used in traditional medicine contain a wide range of ingredients that can be used to treat chronic as well as infe- ctious diseases. A vast knowledge of how to use the plants accumulated in areas where the use of plants is still of great importa-nce (Diallo et al., 1999). The medicinal value of plants lies in some chemical substa-nces that body. The most important of these bioactive compoun- ds of plants are alkaloids, tannins and phenolic compounds (Edeoga et al., 2005). Antimicrobial activities of Saudi Arabian desert plants Mohamed Eldesouky Zain 1 , Amani Shafeek Awaad 2,* , Mounerah Rashed Al-Outhman 1 , Reham Mostafa El-Meligy 2 1 Botany and Microbiology Department, Faculty of Science, King Saud University, Riyadh, KSA 2 Chemistry Department, Faculty of Science, King Saud University, Riyadh, KSA. *Corresponding Author: Email: amaniawaad@hotmail.com Received: 15 October 2011, Revised: 8 November 2011 Accepted: 9 November 2011 Abstract The ethanol extracts of Alhagi maurorum Medic., Chenopodium murale L., Convolvulus fatmensis G. Kunze., Conyza dioscoridis (L.) Desf., Cynanchum acutum L., Diplotaxis acris (Forssk) Boiss, Euphorbia cuneata Vahl., Origanum syriacum L., Solenostemma argel (Del.) Hayne. and Tamarix aphylla L.(Karst) showed significant antimicrobial activity against Gram negative, Gram positive bacteria, unicellular and filamentous fungi. However, Tamarix aphyla showed remarkable activity against Aspergillus flavus and 16, out of 19, strain of the investigated test organisms. The highest MIC value was obtained by Tamarix aphyla against 8, including all the filamentous fungi, of the investigated test strains. However, the extract of Cheno-podium mural showed the best MIC against the unicellular fungi. Keywords: medicinal plants; antibacterial activity; antifungal activity; Alhagi maurorum; Chenopoidum murale; Convolvulus fatmensis; Conyza dioscoridis; Cynanchum acutum; Diplotaxis acris; Euphorbia cuneata; Origanum syriacum; Solenostemma argel; Tamarix aphylla