Brown seaweeds are often employed in single species toxicity testing to study the association between the pollutant and the biota in contaminated marine habitats. We have used Fucus serratus (Phaeophyta) from one Cu resistant and one non-resistant population to evaluate the effect of prior exposure to metal pollution on toxicological endpoints. Analysis of comparative toxicity was conducted for embryo rhizoid elongation and adult relative growth rate (RGR). Algae that had previously been exposed to Cu expressed consistently lower levels of sensitivity to Cu than those that had no history of exposure to the pollutant. For both non-resistant and resistant populations rhizoid length was a more sensitive endpoint than adult RGR. While early life history stages of brown algae are generally regarded as being pollution-sensitive and inhibition of spore and embryo rhizoid elongation is frequently used as endpoints in bioassays, the test results may be affected by prior exposure of the parent algae to the pollutant. We conclude that the effect of prior exposure should be considered when comparing endpoints between studies and when selecting material for future testing.