Macroalgae along the Antarctic Peninsula are generally unpalatable to sympatric herbivores. This is particularly true of the large, brown macroalgae that dominate the communities in terms of cover and biomass. Variation within and between individuals of the ecologically dominant genus Desmarestia has also been detected. In the present study, we examined within thallus variation of both chemical (palatability) and physical (toughness) defences in Desmarestia anceps and Desmarestia menziesii. Each species was divided into the holdfast, the primary stem, and the lateral branches, with tissue fitness values assigned to each component. Lipophilic and hydrophilic extracts were obtained from each thallus component and incorporated into artificial alginate foods. A series of bioassays tested the palatability of these foods to the sympatric, herbivorous amphipod Gondogeneia antarctica. Toughness measurements were also performed on each thallus component. Results from D. anceps, a highly differentiated species, were consistent with the Optimal Defence Theory (ODT) in that the most valuable tissue component (the primary stem) was the most strongly chemically and physically defended. The holdfast, also with a high tissue fitness value, was strongly physically defended. Laterals, which are replaceable and assigned a lower fitness value, were moderately chemically defended. No significant differences were detected in the palatability of the differing thallus parts in D. menziesii, although the holdfast and primary stem were tougher than the lateral branches. These results remain consistent with the ODT as D. menziesii displays less differentiation between the primary stem and the lateral branches and were assigned a similar fitness value. © 2005 Elsevier B.V. All rights reserved.