Phlorotannins are polyphenoloic metabolites occurring only in the Phaeophyceae that have numerous putative primary roles (e.g. cell-wall construction and storage) as well as secondary metabolic roles, which include herbivore feeding deterrence and protection from UV radiation. The proposed role of phlorotannins in the defense against UV radiation is of particular importance in the Antarctic due to depletion of the stratospheric ozone layer in that area. Several studies of brown algae have found evidence of an induction response (the production of defensive metabolites, including phlorotannins) after grazing by various mesograzers, after simulated grazing/wounding, and after exposure to increases in UV radiation. This study aimed to determine if phlorotannin production or other defenses in two dominant, endemic Antarctic species (Desmarestia menziesii Montagne and Desmarestia anceps J. Agardh) could be induced by an increase in exposure to UV radiation or by natural and artificial grazing. An in situ experiment failed to detect any effect of UV radiation on phlorotannin concentrations in either species or on subsequent palatability in feeding bioassays. A laboratory-based experiment did not detect any effect of mesoherbivore grazing or simulated grazing (wounding) on palatability or the concentration of phlorotannins in D. menziesii. Instead, phlorotannin concentrations increased in all treatments in both experiments, consistent with an increase in overall resource availability due to an increase in available PAR compared with the in situ irradiance at the algal collection sites. © 2006 by the Phycological Society of America.