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Stenocereus griseus H. (pitaya de mayo) is a columnar cactus that can be found growing in the arid and semi-arid lands of Mexico. It produces juicy fruits with colored flesh ranging from orange to red- purple. The color of the fruit is due to the presence of betalains, which are water soluble nitrogen-containing compounds. In this study we analyzed fruits of two varieties of S. griseus H.: red (PR) and orange (PN), by measuring their composition in total betalains (betacyanins + betaxanthins), total soluble phenolics, and phenolic acids. The antioxidant activity of the fruits was also determined by DPPH assay and IC50. Total betalains content of red pitaya (PR) was 347.3 +/- 21.0 mg of total betalains (BET)/100 g of dry flesh (DF), while in orange pitaya (PN) it was 215.04 +/- 36.16 mg of BET/100 g DF. Total soluble phenolics (FST) content in the fruits of PR was 166.5 +/- 14.4, and in PN was of 52.8 +/- 3.8 mg equivalents of gallic acid (EAG)/100 g DF. Free phenolic acids (AFL) were found in a higher proportion than those glucosylated and esterified acids. The PN had higher content of AFL than the PR. IC50, which represents the concentration of the extract needed to reduce 50% of the DPPH, for the fruits of S. griseus was 59.8 +/- 0.32 mu M for PR, and 161.7 +/- 4.8 mu M for PN, thus showing a higher antioxidant activity (AA) for PR. The AA in S. griseus is attributed to betalains rather than to phenolics which were present in a lower proportion. S. griseus represents an alternative to increase and diversify the consumption of antioxidants in the population of arid and semiarid lands of Mexico.