After two annual cycles of ecological surveys examining the distribution of mangrove-associated macroalgae in contaminated and uncontaminated estuaries in the Sydney region, south-eastern Australia, two species were selected for examination inlaboratory toxicity tests. The Rhodophyta species, Caloglossa leprieurii and Catanella nipae, were chosen, as they appeared to be highly influenced by ambient contamination concentrations, i.e., the former was distributed more frequently in areas of high contamination, while the latter was distributed more widely in areas of lower contamination. Laboratory toxicitytests were undertaken to examine their sensitivity or tolerance to metal contamination, and to establish their potential as test species in estuarine toxicity bioassays. Due to the predominance of asexual reproduction in these two algal species, physiological endpoints, namely net photosynthesis and respiration rates, were developed. These endpoints have rarely been used in previous macroalgal toxicity tests. Algae for testing were collected from two estuaries containing different pollutant loads; the more contaminated Parramatta River, and the less contaminated Hawkesbury River. In both algal species from both rivers, exposure to copper concentrations greater than 2 μg L-1 resulted in a decline of net photosynthesis and an increase in respiration rate. C. nipae was found to be more sensitive to copper than C. leprieurii collected from the same river estuary. Both algae collected from the Hawkesbury River were more sensitive than the same species collected from the Parramatta River.Overall, the results of this study indicated that physiological endpoints can successfully be used as sensitive endpoints in macroalgal toxicity testing. The results also advocate collecting organisms for toxicity testing from less contaminated sites, in order to gain a toxicity index which protects the majority of that individual species.