The abundance of groundnut-nodulating rhizobia in 32 locations in Sudan with variable soil properties did not correlate significantly to either the soil chemical properties or the duration since groundnuts were last planted in crop rotations;Rhizobium isolates were obtained from six of the commonly grown native legumes in Sudan. The isolates varied in physiological characteristics, serological properties, and nodulating ability on a Sudanese groundnut cultivar. The cultivar showed selectivity for rhizobia isolated from groundnut;Greenhouse efficiency testing revealed that the commercial strain (TAL 309) was more efficient in nitrogen fixation than either of two Sudanese isolates tested (Wad Medani and Kadugli). The host genotype (Virginia or Spanish) little influenced the efficiency of the strains;The Rhizobium strains TAL 309, Wad Medani, and Kadugli varied in colony morphology, which provided a means of strain recognition of nodule isolates. The method was verified by using serological techniques. Results of a competition study among the three strains showed that the order of competitive ability was TAL 309 > Wad Medani > Kadugli irrespective of the cultivar type;Increasing the pH from 6.5 to 8.0 in a pot experiment did not influence the competitive ability of the two Sudanese strains when tested on the Sudanese groundnut cultivar 'Ashford'. Raising the inoculation level of the less competitive, but efficient, Kadugli strain by 10('4) times that of the Wad Medani strain gave the Kadugli strain a competitive advantage and resulted in 100% of the main-root nodules being formed by the Kadugli strain;Under Sudanese field conditions, inoculation of two groundnut cultivars, 'Ashford' and 'Barberton', with selected strains by using different methods did not result in comparable yields to those obtained by adding 120 kg N ha('-1) as ammonium sulfate. Trends did favor the peat carrier over the oil carrier, and placing the inoculant 10 cm below the seeding depth. The serologically distinct inoculant strain, TAL 309 (applied at 5.3 x 10('7) viable cells per two groundnut seeds), was fairly competitive with the native rhizobial population present at 2.1 x 10('4) g('-1) of soil. Approximately 40% of the nodules were occupied by the inoculant strain.