The glutamine synthetase (GS) isozymes in the plant fraction of nodule extracts from 62 cultivars of Phaseolus vulgaris L. and one cultivar of Phaseolus lunatus L. were analyzed by polyacrylamide gel electrophoresis. All P. vulgaris nodule extracts displayed two GS activity bands: a nodule-specific band (GS(n1)) and a band (GS(n2)) similar to the single band (GS(r)) present in root extracts. In nodule extracts of P. lunatus, the GS(n1) band was detected, but the GS(n2) band was barely detectable. In contrast to P. vulgaris, the GS(n2) band and the GS(r) band of P. lunatus appeared to be different. The electrophoretic mobility of the GS(n1) band in P. vulgaris was governed by both the plant cultivar and the development stage of the nodule. In nodule extracts of P. vulgaris and P. lunatus, the zone of GS(n1) activity coincided with six to nine distinct protein bands as revealed after treatment of gels, which had previously been stained for GS activity, with Coomassie blue. All these protein bands were shown to consist of polypeptides of identical molecular weight (approximately 47,000 daltons) by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Our results indicate that P. vulgaris continuously generates isozymes of GS(n1) of increasing electrophoretic mobility during the course of nodule development.