This study tested the hypothesis that the introduced yabbie Cherax destructor Clark, 1936 has the potential to compete with the endemic marron Cherax cainii Austin, 2002 for food resources. Multiple stable isotope analyses were conducted in the Hutt River, Western Australia, in summer (December) and winter (July), 2003. Summer samples indicated that these species occupied similar predatory trophic positions when their assimilated diet consisted of a large proportion of Gambusia holbrooki. Although C. cainii continued to assimilate mostly animal matter based on winter signatures, those of C. destructor appeared to shift towards a more herbivorous trophic position. The study suggests that C. destructor and C. cainii may be keystone species in the Hutt River, possibly altering the cycling of nutrients and structure of the aquatic food web since their introduction into this system. The ecological implications of the continued invasion of C. destructor into the aquatic systems of south-western Australia, particularly with regard to competition with the other endemic freshwater crayfishes, are discussed.