Abstract Aseptically grown seedlings of Trifolium repens were inoculated with either pure cultures of two serologically distinct strains of Rhizobium trifolii or with a 50: 50 mixture. Identification of the strains of nodule bacteria was possible 66 h following inoculation and thereafter, at any time up to the harvest at 10 weeks. The identifications were carried out first on isolates from root hairs and later from direct microscopic observations of mature nodule sections. The antiserum for each strain was conjugated with fluorescein isothiocyanate which gave the specific bacterial smear or nodule centre a distinct bright green colour which was easily distinguishable from the deep blue of the non-specific strain. The experiment indicates how useful the immunofluorescence technique can be for rapid identification of nodule bacteria either in the very early infection stages or from mature nodules. The results clearly demonstrate that individual plants exerted selection for specific bacterial strains but not always for the same strains. The technique is discussed in terms of its use for ecological, bacterial strain competition and host selectivity experiments.