Aims: To determine the impact of medium composition, bacterial strain, trehalose accumulation, and relative humidity during seed storage on the survival of Bradyrhizobium japonicum on soya bean [Glycine max (L.) Merr.] seeds. Methods and Results: Bacteria in liquid cultures were applied to seeds, and the number of survivors was quantified after 2, 24, 48, or 96 h. Addition of yeast extract to a defined medium increased on-seed survival 50- to 80-fold. Addition of 40 mmol l(-1) of NaCl to the medium doubled or tripled the accumulation of trehalose in cells and increased survival several fold, and the addition of both salt and trehalose had an additive effect. There was a threefold difference among strains in survival, and survival of the various strains was significantly correlated with differences in the accumulation of trehalose. The correlation between trehalose accumulation by bacteria and survival was also highly significant in other experiments. Studies in controlled humidity environments showed 100-fold or more differences in survival. Conclusions: The consistently significant correlation of trehalose content of cells with survival on seed suggests that trehalose is an important component of the survival mechanisms. When some of the factors (salt and trehalose in the medium plus humidity control) were studied in combination, several 100-fold increases in survival of bacteria on seeds were recorded. Significance and Impact of the Study: It is possible by manipulation of several parameters - strain selection, salt and trehalose content of the medium, control of relative humidity - to achieve substantial improvements in survival of Bradyrhizobium on soya bean seeds.