A .S . Y A DA V, R .K . V A SH IS H AT , L . D. KU Y KE ND A LL AN D F. M. H AS HE M . 1998. A perturbation of the histidine biosynthetic pathway in legume microsymbionts can abolish their symbiotic competence. Twenty-one histidine-requiring (His − ) mutants were isolated from berseem clover-nodulating, symbiotically-competent (Nod + , Fix + ) Rhizobium leguminosarum bv. 'trifolii' strain RTH 48 Sm r by N-methyl-N?-nitro-Nnitrosoguanidine (MNNG) mutagenesis followed by enrichment. These mutants were analysed for their biochemical defect and the corresponding effect, if any, on their symbiotic abilities. Cross-feeding, supplementation and enzymatic studies identified three types of mutants. Group 1 mutants, His-2 and His-12, grew with histidine supplementation but not with the addition of either L-histidinol or L-histidinol phosphate to the medium ; they lacked histidinol dehydrogenase (EC 1.1.1.23) activity and consequently formed only ineffective, or 'non-fixing' nodules. Group 2 mutant, His-17, grew when supplemented with either L-histidinol or L-histidine, had low histidinol phosphate phosphatase (EC 3.1.3.15) activity (37% of wild-type), and consequently failed to nodulate berseem clover. Group 3, the remaining 18 mutants, grew when supplemented with L-histidinol phosphate, L-histidinol or histidine, and did not nodulate. Typically, reversion rates were between 10 −7 and 10 −8 . Defects in early steps of the pathway abolished nodulating ability, whereas lesions in the last step did not. The last step, however, was required for symbiotic nitrogen fixation. It is hypothesized that histidine may be supplied by the host in sufficient quantity for nodulation by histidinol dehydrogenase mutants to occur, whereas the amount provided in the nodule may be insufficient to support bacteroid development and nitrogen fixation.