To investigate the role of extracytoplasmic function sigma factors in the Rhizobium-legume symbiosis, the genome sequence of Rhizobium leguminosarum bivoar viciae 3841 was analysed using bioinformatics approaches. In total, twenty sigma factors were identified, including sixteen extracytoplasmic function sigma factors. Twelve of the sixteen ECF sigma factors appeared to be co-transcribed with a regulatory gene. The role of the ECF sigma factors in responding to legume produced compounds was investigated by growing strain 3841 containing various ecf reporter plasmids with pea root exudate. Three ECF sigma factors showed different expression patterns when grown with root exudate compared to minimal medium. One of the three sigma factor aenes. ecfR, which was induced by pea root exudates, was investigated further and was shown to be expressed in pea infection threads. Over-expression of ecfR was lethal to R. leguminosarum 3841, but co-expression with the adjacent gene, asfR, prevented the lethal effect.