Surveys conducted after a crude oil spill indicated that the intertidal gastropod mollusc Austrocochlea porcata may be highly sensitive to the pollutant, and therefore also valuable as a biomonitoring organism. Toxicity tests conducted in the laboratory and field established cause-effect for A. porcata mortalities on exposure to environmentally relevant concentrations of crude oil constituents. Glutathione antioxidant system components (glutathione and glutathione peroxidase, GPx) and oxidative damage (lipid peroxidation) in A. porcata were measured to determine whether any of these biochemical parameters showed potential as biomarkers of sublethal oil exposure. GPx was the most promising candidate for field-based biomarker studies after showing a dose-dependent induction to a crude oil water accommodated fraction (WAF) in laboratory assays. However, subsequent manipulative field experimentation indicated that the GPx response was not sufficiently sensitive and not necessarily predictive of population level effects when measured in situ.