The association between sorghum and N 2fixing bacteria has been assessed only under limited conditions. We investigated biological nitrogen fixation (BNF) in situ in fifteen sorghum genotypes with dry or succulent culm types under five edaphoclimatic conditions. One randomized block experiment was established in each of five locations, from the humid to the semiarid regions of Pernambuco state, Brazil. BNF was estimated using the 15 N natural abundance method by comparing the average d 15 N value of each sorghum genotype with those of the reference species. High levels of productivity, up to 22 Mg shoot biomass ha -1 in the 3-month cycle, were obtained where rainfall was high, and up to 5 Mg ha -1 was obtained under low rainfall. The nitrogen contents showed a similar pattern as biomass production, and the genotypes with the highest productivity accumulated from 200 to 300 kg N ha -1 . BNF ranged from 55 to 78% of plant N in one location and from 36 to 56% in another location, but BNF did not occur in the other three locations. Although the factors that blocked effective symbiosis were not determined, symbiosis was not influenced by P or K availability. The proportion of N 2 fixation was similar in the grainproducing, dry culm genotypes and in the sugar-rich, succulent culm genotypes. The sorghum genotypes fixed N 2 , reaching up to 218 kg ha -1 N, without inoculation with diazotrophs. Therefore, sorghum has a high potential to fix atmospheric N 2 , but the factors that block N 2 fixation must be identified for crop management planning.