AN increase in beef cattle production on the 350 million hectares of acid, infertile savannahs of tropical South America would have a major impact on world food production. Production in these regions is limited by the inadequate nutritional value of the native vegetation, and legume-based pastures are now being introduced1 to raise the protein content of available forage. However, to obtain full benefit from biological nitrogen fixation through nodulated legumes it is necessary that the introduced legume form an effective symbiosis with Rhizobium. The low pH, limited availability of phosphorus, and very high levels of aluminium and manganese in these soils, are likely to affect nodulation and nitrogen fixation. It is therefore essential that strains of Rhizobium adapted to acid conditions are used. Alkali production by rhizobia from tropical legumes and the consequent rise in pH of test media has made the development of a simple test to select such strains difficult2. We report here that interference from alkali production can be eliminated by minor changes in the screening medium. This finding challenges the view that alkali production is a distinguishing characteristic with ecological significance3, and is also a starting point for detailed study of the effects of acidity and related factors on growth of Rhizobium.