Biological N2 fixation is performed most effectively by prokaryotic diazotrophs when in mutualistic symbioses with higher plants. The most intensively studied N2-fixing symbioses involve leguminous plants and rhizobia. However, Frankia actinomycetes have attracted attention recently because they form root nodules on a broad range of non-legumes and because such nodules fix N2 as effectively as rhizobial nodules. Since the Frankia symbiosis results from an actinomycetic invasion of plant roots, it has been termed the 'actinorhizal symbiosis'.