Fundamental to the accuracy of stable isotope analysis in trophodynamic studies is the ability to predict discrimination between a consumer and its diet. Despite the widespread use of stable isotope analysis in trophic ecology, uncertainty still surrounds the factors affecting consumer-diet discrimination. Here we present evidence that diet quality and location of muscle tissue analysed affects the consumer-diet discrimination for the western rock lobster, Panulirus cygnus. Consumer-diet δ15N and δ13C discrimination for western rock lobster tail tissue were 1.67–2.97 and 2.92–3.60‰, respectively, with δ13C discrimination differing to values reported in the literature. Differences in nitrogen and carbon discrimination were observed between tail and leg tissue of lobsters of 1.22 and 1.13‰, respectively. Diet quality was also found to affect consumer-diet discrimination, with high protein pilchard diet leading to lower δ15N and higher δ13C discrimination. Diet quality should be considered as a factor that has the potential to affect consumer-diet discrimination when interpreting results from stable isotope studies.