The influence of two strains of the mycorrhizal fungus Paxillus involutus on the development of micropropagated Alnus glutinosa plants was investigated. Mycorrhizal synthesis between the P0 and Mi strains and previously rooted plants of A. glutinosa has been obtained in monoxenic conditions. Mycorrhizal symbiosis has beneficial effects on different quantitative and qualitative aspects of the plant development. Both strains lead to an improved growth response of the plant height and leaf area. P0 was more effective than Mi in improving height of the stem. Both Paxillus strains lead to larger leaves and total leaf area than that observed on the control plants and was increased to almost the same extent by both strains. Finally, more lateral bud breaks were observed on plants cocultivated with the P0 strain, it seems possible that the P0 strain stimulates the ramification of the upper part of in vitro plants.