This study was conducted to examine the effect of simulated acidic precipitation and aluminum treatment on the growth of Alnus species and the contents of nitrogen fixed by Frankia. The growth and nitrogen contents of Alnus species inoculated with nitrogen-fixing organisms were greater than those of noninoculated Alnus species when treated with simulated acidic precipitation. The nitrogen contents were higher in the soils used for the growth of Alnus glurinosa inoculated with Frankia than in those without Frankia inoculation when treated with either acidic precipitation or aluminum. The-development of root hairs treated with simulated acidic precipitation was poor as lhe pH level decreased, and the injury of A. glurinosa was more severe than that of Alnus hirsuta under scanning electron microscopy (SEM). The growth and nitrogen contents of A. hirsura inoculated with Frankia were greater than those of non-inoculated species and A. glutinosa when treated with aluminum. The leaves of A. glutinosa became yellowish-brown and fell earlier. Under SEM and light microscopy, the surface layer of roots in both Alnus species was injured severely and the number of root hairs decreased as aluminum levels increased.