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The efficiency of different Finnish Frank& strains as symbionts on Alnus incana (L.) Moench was evaluated in inoculation experiments by measuring nitrogen fixation and biomass production. Since all available pure cultures of Frankia are of the Sp type (sporangia not formed in nodules), but the dominant nodule endophyte of A. incana in Finland is of the Sp § type (sporangia formed in nodules), crushed nodules of this Frankia type were included.The Sp-pure cultures, whether originating from A. incana or A. glutinosa, produced with one exception, similar biomass with A. incana. The highest biomass was produced with an American reference strain from A. viridis crispa. Using Sp § nodule homogenates from A. incana as inoculum, the biomass production was only one third of that produced by Sp pure cultures from the same host.Hence, through selection of the endophyte it is possible to exert a considerable influence on the productivity of Alnus incana.