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Bush bean plants were used to study water use efficiency (WUE) and total dry matter (TDM) production for well-watered, moderately watered, and water-stressed plants grown in the open, and inside covers of perforated and non-perforated plastics. WUE increased quadratically (0.83 less than or equal to R-2 less than or equal to 0.98) with days after plant emergence to harvest for the three soil moisture regimes under each covering treatment. The water-stressed plants in the open and under the closed plastic attained the maximum and the minimum WUE, respectively. TDM increased quadratically with time following plant emergence (0.88 less than or equal to R-2 less than or equal to 0.99). The water-stressed plants had the minimum TDM at the time of pod formation (55 days). These results were attributed to the modified microclimates inside the perforated and closed plastic covers compared with the open. Stomatal conductance was lowest inside the closed plastic and highest in the perforated enclosure, with intermediate values in the open. The temperature difference between leaf and ambient air was significantly higher for the open compared with the covered treatments. The leaf-to-air vapour pressure deficit was lowest for well-watered plants under closed cover and highest for those in the open. Well-watered and moderately stressed bush bean produced the highest TDM inside perforated plastic covers during the pod formation stage. This suggests that perforated plastic covers might be used to enhance bush bean production in semi-arid regions. (C) 1997 Elsevier Science B.V.