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A 12-year study examined the diets of red kangaroos (Macropus rufus), domestic sheep (Ovis aries) and wild European rabbits (Oryctolagus cuniculus) on open plains in the arid, shrub rangeland of southern Australia. Also examined were levels of dietary overlap between these animals and their foraging in relation to resource availability. The potential for interspecific competition in different conditions was examined. Insights were also sought into the impact of these wild and domestic herbivores on the arid zone pastures. The diet of red kangaroos was based around grasses, except in wet seasons when forb intake was high. In dry seasons shrubs were also eaten but in severe drought grass intake was almost 90%. The diets of sheep showed a similar pattern but were more variable. While grass was important in their diet, they ate more forbs in wet times and much more shrub in dry conditions than the kangaroos. However, in drought they had a high dietary overlap with the kangaroos. Rabbits had broad diets in good conditions but ate much browse (bark, twigs and roots) in drought. The data pointed to marked competition between the herbivores, with sheep impacting more on kangaroos when pasture conditions began to deteriorate. A decline of perennial shrubs in these arid rangelands was considered probable because of the emphasis of sheep and rabbits on them in dry times; the dominant chenopod shrubs do not survive persistent heavy grazing.