Cells of the root nodule bacterium Rhizobium leguminosarum bv. viciae WSM710 were cultured in a medium containing 20 mu M Fe-57. Mossbauer spectra of the cells at 5.5 and 3.7 K indicated that the major form of iron present in the cells was in the form of polynuclear iron(III) clusters. At 5.5 K the spectral component associated with these clusters was in the form of a superposition of a broad feature (large magnetic hyperfine field distribution) and a doublet. On lowering the temperature of the cells to 3.7 K, the spectral component was transformed into resolved magnetic hyperfine field splitting which yielded a magnetic hyperfine field of 42.4 T when fitted with broad Lorentzian peaks. These spectral characteristics are typical of the hydrated iron(III) phosphate cores of several bacterioferritins. A small fraction (11%) of the Mossbauer spectral area of the cells was in the form of a doublet which yielded parameters (delta = 1.35 mm/s; Delta E-Q = 3.15 mm/s) indicative of iron(II). The parameters are very similar to those of a spectral component previously observed in several other microbes (R. Bohnke and B.F. Matzanke (1995) BioMetals 8, 223-230) and which has been associated with a 2.2 kDa oligomeric iron(II) carbohydrate phosphate.