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Changes occurring during fruit ripening and duration of fruit development were studied in Selenicereus megalanthus (Scum. ex Vaupel) Moran (yellow pitaya), a climbing cactus grown in protected structures at three sites in the Israeli Negev desert. During ripening, peel color turned from green to yellow, fruit dimensions slightly changed, and pulp content markedly increased. Total soluble solids and soluble sugars in the pulp increased, while starch content decreased. Acidity decreased at the last stage of ripening. Fruit in which most of the peel area had turned yellow (stage 4) were given the highest taste grade by a panel of tasters. Measurements of ethylene and CO2 evolution indicated that fruit was nonclimacteric. The mean number of days from anthesis to fruit of stage 4 was negatively correlated with the mean of the maximum and the minimum temperatures during the growth period. Daily accumulation of heat units (HUs) was calculated as the difference between daily mean temperature and a base temperature of 7 degrees C. Sum of HUs for the period from anthesis to ripening was 1558 +/- 2 HUs.