The presence of derivatives of the broad host range plasmid RP4 in strains of Rhizobium leguminosarum biovar viciae severely inhibited nitrogen fixation by these strains in nodules on cultivars of pea (Pisum sativum). The strains formed small white nodules. Yield and total nitrogen values were comparable with those obtained for plants inoculated with a non-nodulating mutant. Strains carrying the same derivatives gave rise to nitrogen fixing nodules when inoculated on cultivars of lentils (Lens culinaris). Similar results were observed with plasmid R702 but not with R751, suggesting that the effect is limited to plasmids of the IncP alpha classification. Histological examination of nodules induced by strains carrying RP4 indicated that there are fewer infected cells and starch granules are organised unusually in the infected cells. Tn5 mutagenesis of plasmid RP4-4 was undertaken and Tn5 inserts were screened for abolition of the effect on nitrogen fixation. Eight mutants, having no effect on nitrogen fixation, were isolated. Seven of these had lost the ability to transfer by conjugation and the eighth was greatly reduced in conjugation frequency. Physical analysis of the transposon inserts revealed that they were located in the Tra regions of RP4.