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Five sympatric species of perennial pasture plants were studied in monospecific plots at an experimental site in the arid zone of southern Tunisia. For one year following establishment, irrigation was supplied monthly to test plots so as to complement natural precipitation and simulate an "optimum" rain year (250 mm of precipitation occurring, evenly distributed, between September and April). In addition to hydrometric fluctuations in the upper soil layers, growth rate and phenological activity were monitored for all species (Cenchrus ciliaris, Digitaria commutata, Stipa lagascae (Poaceae), Argyrolobium uniflorum (Fabaceae) and Plantago albicans (Plantaginaceae)). Overall growth rate and foliar extension measurements revealed differential water use efficiency, phenology, and response to water stress in the species studied. C. ciliaris and D. commutata were the most efficient in their use of water. The former, a C4 grass species of paleo-tropical origin, was primarily active in spring and autumn. D. commutata, along with the remaining three species, all of Mediterranean origin and of the C3 type, was active primarily in winter Stipa lagascae and Argyrolobium uniflorum appear to belong to the "arido-active" group of drought-tolerant plants, continuing to insure photosynthesis until soil water potential drops to below -5 MPa. However, their overall productivity was low as compared to the two arido-passive species, Cenchrus ciliaris and Plantago albicans. The five species studied were thus complementary in their utilization of soil water supplies and their response to water shortages. Such information may serve in determining appropriate species mixtures and planting densities to be used for the restoration of self-perpetuating pastoral ecosystems in the arid zones of North Africa.