Rhizobium leguminosarum strains were produced for the biological control of Sitona larvae by introducing Bacillus thuringiensis subspecies tenebrionis delta-endotoxin genes (cryIIIA). Comparisons between a transgenic and parent strain show that transformation has induced changes not associated with the intended function of the transgene. Although growth rates in laboratory cultures are similar for both strains the ability to compete for nodule sites is greater in the transgenic than in the non-transformed parent strain, a character that has remained stable over 4 years. This increased ability, which was previously observed in axenic culture, is shown here to also occur in non-sterile soil, although the effect is less pronounced than in sterile conditions. Experiments in soil show a highly significant difference from the expected nodule occupancy ratio, assuming no difference between genotypes and with no significant variation between replicates. These results demonstrate that the ecological and agronomic characters of transgenics might be unexpectedly altered by transformation. Such characters might have a bearing on the safety and/ or success of transgenics released into the environment.