The addition of combined nitrogen to substrate at an appropriate rate can stimulate N2-fixation thus inreasing the efficiency of the Alnus-Frankia symbiosis. To examine how nitrogen additions can effect the peformance of different pairs of symbionts, growth and time course of N2-fixation were studied in plants supplied with NH4NO3. Two cloned ofAlnus glutinosa (L.) Gaertn., propagatedin vitro, were inoculated with two strains ofFrankia (AVP3d and ACN14a) and grown in a greenhouse. Calcined montmorillonite (TotfaiceR) was used as growth substrate. Six N treatments were made up of varied amounts of NH4NO3 supplied in one single addition shortly before inoculation. Weekly measurements of shoot height and repeated measurements of nitrogenase activity (acetylene reduction) performed on intact root systems were used to monitor the development of the symbioses. Nitrogen treatments containing from 0.10 to 0.68 mg N g−1 dry substrate stimulated N2-fixation as well as growth. The relative performance of the two clones was different according to N treatment; one clone showed a greater benefit from the nitrogen input. Our results support the recommendation that selection of symbionts according to performance should be carried out with an input of combined nitrogen. This can provide optimum conditions for the development of each pair of symbionts.