Aims: To analyse the symbiotic variations within indigenous populations of rhizobia nodulating red clover (Trifolium pratense L.) in soils of northern Norway and Sweden at different times of the growing season. Methods and Results: A total of 431 nodule isolates sampled under field conditions in summer and autumn, were characterized genetically by targeting both chromosomal and symbiotic genes. The Enterobacterial Repetitive Intergenic Consensus polymerase chain reaction (PCR) fingerprinting of chromosomal DNA revealed considerable variation within the isolated populations that was more influenced by geographical origin than sampling time. Analysis of PCR amplified nodEF gene on the symbiotic plasmid by restriction fragment length polymorphism revealed a high proportion of nod types common to the two studied sites. The symbiotic efficiency of the isolates, representing both dominating and rare nodEF genotypes, showed high N-2 fixation rates in symbiosis with the host plant in a greenhouse experiment using the N-15 isotope dilution method. Conclusions: Effective N-2-fixing strains of Rhizobium leguminosarum bv. trifolii nodulating red clover are common and genetically diverse in these northern Scandinavia soils. Significance and Impact of the Study: This study provides information on the variability, stability and dynamics of resident populations of rhizobia nodulating red clover in Scandinavian soils which has practical implications for applying biological nitrogen fixation in subarctic plant production.