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The knowledge of crop evapotranspiration (ETc) is very important for efficient water use in irrigated crops, mainly in arid and semi-arid regions, where the use of saline water is common. The objective of this study was to determine watermelon crop evapotranspiration (cv. Mickylee) in plants submitted to different irrigation water salinity levels. The experiment was carried out at the Experimental field of the Semi-Arid Federal Rural University, Mossoro, RN, Brazil. The irrigation water salinity levels were of S1 = 0.55; S2 = 1.65; S3 = 2.35; S4 = 3.5 and S5 = 4.5 dS m(-1), with the treatments set up in randomized blocks with four replications. The determination of reference evapotranspiration (ETo) was accomplished by FAO-Penmam-Monteith method, and ETc was determined by two weighting lysimeters installed in the treatments S1 and S5. The results showed that watermelon ETc decreases with the increase of salinity in the irrigation water. The total ETc during the evaluated period was 245 and 214 mm, respectively for S1 and S5. The Kc medium values obtained for growth stage were: 0.23, 0.68, 1.12 and 0.90 for water S1, and 0.24; 0.61; 0.98 and 0.78 for S5.