A recombinant plasmid (pcPvNGS-01) containing sequences related to glutamine synthetase (GS) has been identified from a cDNA library constructed from poly (A)+ RNA isolated from root nodules of Phaseolus vulgaris L. The identification of this recombinant relied on the observations that: (a) the clone hybridized strongly to purified GS mRNA; (b) in hybrid-select translation experiments, the clone selected mRNA that produced a polypeptide identical in molecular weight to purified GS subunits which was immunoprecipitated with anti-GS-antiserum; and (c) the translated nucleotide sequence of the cloned cDNA was homologous to a partial amino acid sequence of higher plant GS. The cloned cDNA hybridized to poly (A)+ RNA of different mobilities from leaves, roots, and nodules of P. vulgaris. In RNA "dot" blots washed at different stringencies, differences were observed both in the relative amounts of GS mRNA in different tissues and in the strength of their hybridization to the cDNA probe. The cloned probe hybridized to several fragments of restricted P. vulgaris DNA but not to DNA from Rhizobium phaseoli. These results suggest that GS is coded for by a small multigene family showing organ-specific expression.