Soils from a well controlled field experiment were screened for the presence and number of cells of Rhizobium leguminosarum bv. trifolii capable of effectively nodulating the host plant, white clover (Trifolium repens). Soils had been amended with anaerobically-digested or undigested sewage sludge at rates of 0, 100 and 300 m3 ha −1 yr−1on plots of differing pH since 1980 and up to the present. Applications of anaerobically-digested sludge included additions with or without heavy metal salts. Rhizobium were present in all of the treatments, apart from the most metal-contaminated treatment in the soil of lower pH, despite the absence of the host plant from the field sward. Lack of nodulation and nitrogen fixation (acetylene reduction activity) for T. repens growing in soils was, in some cases, probably caused by the high concentrations of extractable nitrate present as plants subsequently grown in N-free media were effectively nodulated. Important effects on the size of the effective rhizobial population were apparent in relation to the soil pH, sludge type and addition rates, and the concentration of heavy metals present.