The effects of nickel on hydrogen uptake and the post-translational processing of the large subunit of the hydrogenase protein in three Frankia strains (one isolated from an Alnus-Frankia symbiosis and two from Casuarina-Frankia associations) were investigated. All three strains responded to the addition of nickel with an increase in hydrogen uptake. Additional nickel did not affect nitrogenase activity, however evolved hydrogen was detected in Frankia KB5 in the absence of additional nickel, indicating that hydrogenase was not active. No increase in the processing rate of the hydrogenase large subunit was found with increasing nickel concentrations for any of the strains, indicating that the strategy for regulating hydrogenase in Frankia is different from that in other microorganisms.