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Growth, photosynthesis and water use efficiency of two Sahel dominant C-4 grasses: Schoenefeldia gracilis Kunth and Dactyloctenium aegyptium (L) P. Beauv were studied under well-watered and drought conditions. D. aegyptium had a faster rate of development than S. gracilis and responded to water-deficits by decreasing leaf area in the earlier stages of development. However, under water-stress the overall reduction of leaf area suffered by S. gracilis was greater than in D. aegyptium. The decrease in net photosynthesis observed under water-deficits resulted mainly from stomatal closure. Analysis of correlation between long-term water-use efficiency and several physiological, structural and growth characteristics shows that long-term water-use efficiency in D. aegyptium was positively correlated with relative growth rates. The reduction in growth resulting from water-deficits was compensated by an increase in the short-term water-use efficiency associated with stomatal closure. In S. gracilis, the long-term water-use efficiency under moderate water-deficits increased but was not positively correlated with relative growth rate; it was associated with a high allocation to non-photosynthetic (support and vascular) tissues. The possible implications of this behaviour for the different ’strategies’ for plant survival in the arid climate of the Sahel are discussed. (C) 2000 Academic Press.