The GC chromatograms (Figs. 2 and 3) show that whilst the common plant palmitic, stearic, oleic, linoleic and linolenic acids are present in both H. polyrhizus and undatus, linoleic acid, an essential fatty acid, predominates. Both pitaya varieties exhibit two oleic acid isomers. In H. polyrhizus straight oleic constitutes 21.6% and its isomer; cis-vaccenic acid at 3.1%. The content of oleic and cis-vaccenic acid in H. undatus is 23.8% and 2.8%, respectively. The non significant difference of the content and proportion of oleic and its isomeric (cis-vaccenic acid) form reaffirms genetic similarities of the varieties