Introduction Medicinal plants are still considered to be a source of new chemical entities with unique chemical structures and pharmacological properties. The same plants growing in different geographical locations under varying climactic conditions can and do differ in their phytochemical constituents (Benetis 2008, Li 1994, Pandita 1983), both qualitatively and quantitatively, and therefore may also exhibit variation in biological activities. It is therefore prudent to screen plants from various geographical locations for both phytochemical and biological activities. Use of plants for medicinal purposes may sometimes differ from place to place and are very often used for the same clinical condition in many parts of the world. This study screened 150 plants collected from different parts of Saudi Arabia for twelve phytoconstituents and a number of biological and antimicrobial activities. Partial and limited information on several of these plants was reported in Fitoterapia (LIV:1, 2 and 3).