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Much of the semi-arid, native rangelands Pilbara region of NW Australia is used for cattle production with the most productive grasslands for cattle grazing dominated by the C-4 perennials grasses; Astrebla pectinata, Themeda triandra, and Eragrostis xerophila. In both grazed and ungrazed plots, we quantified the seasonal variation in the natural abundance of C-13 and N-15 of green foliage. We assigned the C-4 pathway of individual species (NAD-ME vs. NADP-ME) on the basis of delta C-13. Foliage delta C-13 varied with seasonal conditions that in turn dictated patterns of growth (and of photosynthate production), and possibly also with the ’leakiness’ of the bundle sheath cells. However the significant differences present in grazed and ungrazed plots for Astrebla and Themeda, indicate that grazing may result in greater water use efficiency in these semi-arid rangelands. Seasonal patterns of delta N-15 were highly variable and exhibited little correlation with seasonal patterns of growth and periods of inorganic N uptake. While it is possible that N obtained from sources other than the simple uptake of inorganic N contributed to observed delta N-15 patterns, there is little additional evidence to support this hypothesis. There was no difference in delta N-15 between grazed and ungrazed treatments for any of these grasses. (C) 2016 Elsevier B.V. All rights reserved.