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A study was made of cattle grazing three plant communities in a 40 ha area of semi-arid rangeland in central Australia after the opening summer rains. The botanical and chemical composition of the diet selected was examined in relation to the site of grazing and the growth of the animals. Site af grazing and liveweight change in 12 Shorthorn yearling steers were recorded at two-week intervals for 24 weeks. Simultaneously diet samples were collected from a group of four similar cattle which had established oeso- phageal fistulas. The highest liveweight gains (1.2 kg per head per day) were recorded at times when green grass and forb species made up most of the diet. At such times there were peak levels of nitrogen and digestible organic matter in the diet and minimum levels of cell wall constituents. Faecal nitrogen showed a similar pattern to dietary nitrogen. Mean rate of liveweight gain shown by the cattle on these native rangelands over 126 days of growth was 0.79 kg per head per day and over the entire experimental period 0.6 kg per head per day. These gains are quite high and are discussed in terms of the digestible energy available to grazing cattle. Comparisons are made between this arid zone situation and that of native and improved pastures in humid tropical and temperate areas. The relationships between nutrients in the diet, or faecal nitrogen, and the liveweight change of cattle provided a means of determining the quality of the forage available and/or the degree of liveweight change of range land cattle.