Root nodule structural organisation of nine pea Fix^- mutants representing seven symbiotic loci has been studied. This set of mutants has revealed lesions at four developmental stages in the pea-Rhizobium symbiosis. (i) Mutant RisFixA is affected in infection thread differentiation in nodule tissue, infection droplet formation, bacteroid differentiation and nodule persistence. (ii) Allelic mutants RisFixL and RisFixO (sym32) are blocked in bacteroid differentiation. (iii) Mutants RisFixM (sym26), RisFixN, RisFixQ (sym27), RisFixT (sym26) show premature degradation of symbiotic structures. (iv) Mutant RisFixV shows similar defects in nodule persistence but, in addition, this mutant is characterised by an abnormal and severe increase in the thickness of infection thread walls during the process of infection thread 'maturation' and senescence. Combining our new data with previously published studies of pea mutant phenotypes has allowed us to create an integrated scheme for the sequential functioning of the late pea symbiotic genes identified and characterised to date. (C) 2000 Elsevier Science Ireland Ltd.