The stages in the nodulation process that determined the competitiveness of R. leguminosarum bv. trifolii (Rlt) strain 20–15, which proved to be highly competitive for nodulation in Iceland fields tests over several years, is analysed. White clover (Trifolium repens L.) roots were inoculated with inoculum mixtures containing three strains (Rlt 20-15, Rlt 8-9 and Rlt 32-28) in different proportions and cell densities. Competitiveness in root colonization, formation of infection threads and nodule development was assessed for Rlt 20-15 and its weakest competitor, Rlt 32-28. ERIC-polymerase chain reaction (PCR) DNA fingerprinting was used to identify inoculated strains recovered from root surfaces and individual nodules. GFP or DsRed tagged strains were used to determine identity in root hairs and nodules. Both strains colonized the root equally at all inoculum ratios tested. But, Rlt 20-15 initiated significantly more infection threads and formed more nodules than Rlt 32-28. These results show that Rlt 20-15 expresses its nodulation competitiveness during infection, either at infection thread initiation or during successive growth in the infection threads. The data presented support earlier observations that this strain competed well in the field in spite of its inferior ability to survive in the soil.