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Controlled deficit irrigation (CDI) has been studied in a potato crop cultivated in a semi-arid zone (Albacete, Spain). Ten drip irrigation treatments were differentiated by the level of fulfilment of the water requirements. The effect of deficit irrigation at three crop stages (growth, tuber bulking and ripening) has been studied. Tuber yield and its components were highly influenced by the total volume of irrigation water. The treatments with deficit during the last part of the cycle have had the lowest productions. The larger potatoes were obtained in the treatments which had not undergone deficit in the ripening period. Against this, the smallest potatoes were obtained in the treatments with deficit in growth period due to a higher number of tubers per plant. The effect of water on tuber size relies on the combination of deficits during the growth and the ripening stages, through the influence of the number of tubers per plant. The mathematical function that better fits the production obtained with the water volume received is a second-degree polynomial. The irrigation regime that leads to moderate deficit in the beginning of the season (growth and tuber bulking periods) can achieve production results of the order of those obtained by the test treatment, well irrigated along the whole cycle, it being a clearly advisable regime. On the other hand, the least advisable regimes are those that lead to deficit in the ripening stage as well as at growth or tuber bulking. (C) 2001 Elsevier Science B.V. All rights reserved.