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Here we report the variation of diterpenes from three populations of the brown alga Dictyota mertensii. The populations were analyzed by 1H NMR and GC-MS techniques. Both chemical GC-MS and 1H NMR profiles of D. mertensii presented diterpenes as major constituents. In all populations, we detected the prenylated guaiane diterpenes, dictyoxide, pachydictyol A, isopachydictyol A, dictyol B and its acetate, and dictyol C. Although dictyol H has been described as a taxonomic marker, this diterpene was found in only two of the three studied populations. This result reinforces the need for studies of temporal and geographic variation for the establishment of the taxonomic limits of the species chemical patterns. It was also possible to observe mass spectra fragmentation and proton chemical shift in the NMR spectra characteristic from xeniane diterpenes, for the first time in this species, which led to the detection of dictyodial, suggesting that D. mertensii belongs to the chemotaxonomic group of species D. ciliolata, D. jamaicensis, and D. menstrualis , all of which are producers of diterpene from chemical groups I and III. Image 1 • The diterpenes from Brazilian populations of Dictyota mertensii were studied. • D. mertensii is a producer of diterpenes group I and III. • Dictyol H may not be detectable in some populations. • The chemical and its composition might be subjected to environmental conditions. [ABSTRACT FROM AUTHOR]