Larvae of the weevil Sitona spp. specifically eat the root nodules formed on legume plants by the soil bacterium Rhizobium. This can adversely affect the nitrogen fixing activity in the root nodules and lead to decreases in yield. Transgenic rhizobia were used in a novel approach to the biological control of Sitona. Two transcriptional fusions were made in which the coding sequence of the insecticidal crystal protein gene (cryIIIA) from Bacillus thuringiensis subsp. tenebrionis was fused to either the promoter of the rhizosphere enhanced rhiA gene from R. leguminosarum biovar viciae, or the promoter of the nodule specific nifH gene from R. leguminosarum bv. trifolii. The two chimeric genes were transferred to R. leguminosarum bv. viciae and R. leguminosarum bv. trifolii, respectively. Bioassay systems which allowed easy inspection of the plant root systems were used to test the transgenic strains for their ability to protect their host plant from damage by larvae of Sitona flavescens. In both white clover and pea plants nodulated by the transgenic rhizobia a slightly smaller proportion of root nodules were damaged compared to the wild type control plants. In some of the bioassays this was accompanied by increased nitrogenase activity, but it was not reflected in increased plant growth, which was similar in both treatments. These results suggest that a toxin with much higher toxicity against Sitona larvae is necessary for this approach to biocontrol to succeed.