Japanese mountain alder (Alnus hirsuta) seedlings were raised in a factorial combination of two levels of CO2 and two levels of nutrient to examine their foliar defense traits. Alders are actinorrhizal plants and are capable of fixing atmospheric nitrogen through root nodules formed by the actinomycete Frankia sp. We examined the defense capacity of alder in relation to atmospheric CO2 enrichment and soil fertility as simulated acid nitrogen deposition. In order to evaluate the degree of defense in alder leaves under these conditions, a bioassay study was carried out with wild silkworm, "Erisan" (Samia cynthin ricini). While the longevity of Erisan fed with leaves grown at high CO2 and infertile condition was greater than in other treatments, all individuals tested, however, were dead before they reached the pupal stage. The results suggest firstly that the activity of the symbiotic Frankia sp. may have been enhanced by the greater amount of photosynthates that they received from the host plants (i.e. alder) at high CO2. In this process, the Frankia sp. would increase the amount of nitrogen to leaves, which improved the food quality for Erisan. However, the reason that Erisan failed to become pupa could be that some essential elements in alder leaves might be insufficient or that the presence of defense chemicals as total phenolics in leaves might have interfered with Erisan's development.