Rhizobial isolates from the nodules of soya bean (Glycine max (L.) Merr.) grown at three west African locations (in Nigeria and Niger) were examined for colony morphology, serological reactivity, symbiotic effectiveness and tolerance of environmental stresses. Different proportions of two colony types, designated 'wet' and 'dry', were found at each location. The wet and dry strains were delineated in terms of their serological properties, consistent with the likelihood that these colony types represent fundamentally different types of rhizobia. Relatively few of the isolates were effective on soya bean cv. Wilkin, in comparison with a group of isolates from Colombian soils and with a diverse collection of strains. The wet IRj strains were more tolerant of the presence of antibiotics and of NaCl in the culture medium, whereas location of origin determined tolerance of high temperature. The data show that, although they are few in number, rhizobia highly effective with US-bred cultivars of soya do exist in west African soils and offer a useful new source of germplasm for inoculants in tropical environments. The data are discussed in terms of the need for the inclusion of the wet type in the formal taxonomic description of Bradyrhizobium japonicum, otherwise this novel source of effective stress-tolerant rhizobia may be mis-identified and discarded.