Genetic instability within strains of rhizobia maintained on laboratory media is well recognized, although rarely has the mutation been characterized. Variability within a strain introduced into the field is very difficult to recognise due to poor understanding of naturally-occurring populations of rhizobia. We have examined populations of Rhizobium leguminosarum bv. trifolii from both laboratory cultures and field populations and found significant variation in symbiotic effectiveness within both. In Australia, the only significant introduction of Bradyrhizobium japonicum has been strain CB1809 (=USDA136b). Symbiotic tests on field reisolates obtained by plant entrapment indicate little or no change in symbiotic effectiveness up to nine years after introduction. The RFLP pattern, using the RSα probe (Hahn and Hennecke, 1987a) was unchanged but marked differences in serological characters were observed.