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Effects of moisture stress during different stages of cotton development were examined in two experiments when temperatures were mild and evapotranspiration less than 6 mm./day. Two varieties of upland cotton, Acala 4-42 and Deltapine Smoothleaf, were affected similarly. Moisture stress at the beginning of flowering reduced growth rate and the numbers of flowers and bolls. During the second half of the flowering period it reduced the percentage of boll retention, boll number, boll weight, seed index, lint index and lint length. Stress during boll development had similar effects and caused earlier maturity. Lint yield was reduced significantly by moisture stress during each of the periods but tensile strength of lint was not affected. Three irrigations, if properly timed so that no appreciable moisture stress occurred, were sufficient for high yields and good quality of lint, and the first could be postponed until after flowering began without any loss in yield.The problem of optimal timing of cotton irrigation has been investigated by several workers. Harris and Hawkins (1942) and Hamilton et al. (1956) found in Arizona that early irrigations, which induced good vegetative growth before fruiting, also increased cotton yields. However, Thornton (1961) found that cotton needed a good supply of moisture during the boll setting period, until about three-fourths of the bolls were mature, whereas moderate drought in the early growing period did little harm. In the experiment reported by Amemiya et al. (1963), all irrigations in the higher yielding treatment were given after the first bloom appeared. Bruce and Shipp (1962) found that the best moisture regime for cotton consisted of a good moisture supply up to three weeks after the commencement of flowering. Experiments in Greece (Christidis and Harrison, 1955) also showed that the three weeks after the beginning of flowering were the most critical period with regard to moisture. Marani and Horwitz (1963) found that the optimum time to apply a single irrigation to cotton was at the beginning of flowering, but the results of Bruce and Romkens (1965) indicate the importance of good moisture conditions during the four weeks after the beginning of flowering.In San Joaquin Valley, California, Stockton et al. (1967) reported that delaying the first irrigation until the beginning of flowering retarded growth and maturity, but increased the yield of cotton, and similar results were reported by Levin and Shmueli (1964) in Israel. Adams et al. (1942) found that an irrigation applied seven weeks after the initiation of flowering was too late to have any beneficial effect on yield. This was confirmed by Bielorai and Shimshi (1963), who found that stopping irrigation 6-7 weeks after first bloom did not affect cotton yield