The elm spanworm,Ennomos subsignarius(Hübner), is known to be more vigorous and develop faster when reared on juvenile rather than mature leaves of the hickories,Caryaspp., and oaks,Quercusspp., two of its principal hosts in the Southern United States. Seasonal changes in the quantities of two groups of insect nutrients—fatty acids and sterols—were therefore determined in the foliage of pignut hickory,Carya glabra(Mill.) Sweet, and southern red oak,Quercus falcataMichx. var.falcata. In decreasing order of concentration, the major fatty acids found in both species were linolenic, palmitic, linoleic, oleic, myristic, and stearic. Except for the latter two, which remained consistently low throughout the growing season, these fatty acids generally decreased in concentration with the maturation of leaves of both species. In both species, concentrations of sterols varied little throughout the growing season except in foliage only a few days old. Sitosterol was the dominant sterol in both species. Stigmasterol and campesterol were present in relatively low and equal concentrations. Fatty acids in the foliage of tulip-poplar,Liriodendron tulipiferaL., which is not a host of the spanworm, followed the same pattern as in the two host species, but the amounts were twice as great.