Although herbivory by fishes is recognized as a fundamental process structuring coral reef communities, the susceptibility of macroalgae and potential effects of this process at Rocas Atoll are unknown. Macroalgal consumption was evaluated through field transplant assays onto reef sites to investigate the differential susceptibility of species to herbivory by fishes. Of 13 species examined, the red macroalga Digenea simplex was highly preferred and probably constitutes the most important food resource to herbivorous fishes in almost all parts of the atoll. The green macroalga Bryopsis plumosa and the brown algae Dictyopteris jamaicensis , Dictyota crispata , D . ciliolata , D . cervicornis , D . menstrualis , D . mertensii , D. pfaffii and D . pinnatifida were less preferred, probably due to chemical defenses, since all are known to produce secondary metabolites, many of which are broad spectrum feeding deterrents against herbivores including reef fishes. Finally, the algae Sargassum polyceratium and Gelidiella acerosa were less preferred, probably because of both morphological features and chemical defenses. This differential susceptibility of the seaweeds to herbivory associated with different spatial herbivory pressure may be responsible for the variable distribution and abundance of seaweeds around the Rocas Atoll reef.