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The grass flora of Namibia (374 species in 110 genera) shows surprisingly little variation in delta(13)C values along a rainfall gradient (50-600 mm) and in different habitat conditions. However, there are significant differences in the delta(13)C values between the metabolic types of the C4 photosynthetic pathway. NADP-ME-type C4 species exhibit the highest delta(13)C values (-11.7 parts per thousand) and occur mainly in regions with high rainfall. NAD-ME-type C4 species have significantly lower delta(13)C values (-13.4 parts per thousand) and dominate in the most arid part of the precipitation regime. PCK-type C4 species play an intermediate role (-12.5 parts per thousand) and reach a maximum abundance in areas of intermediate precipitation. This pattern is also evident in genera containing species of different metabolic types. Within the same genus NAD species reach more negative delta(13)C values than PCK species and delta(13)C values decreased with rainfall. Also in Aristida, with NADP-ME-type photosynthesis, delta(13)C values decreased from -11 parts per thousand in the inland region (600 mm precipitation) to -15 parts per thousand near the coast (150 mm precipitation), which is a change in discrimination which is otherwise associated by a change in metabolism. The exceptional C3 species Eragrostis walteri and Panicum heterostachyum are coastal species experiencing 50 mm precipitation only. Many of the rare species and monotypic genera grow in moist habitats rather than in the desert, and they are not different in their carbon isotope ratios from the more common flora. The role of species diversity with respect to habitat occupation and carbon metabolism is discussed.