When seasonal variation of caste proportions was quantified in “nests” of the subterranean termiteReticulitermes flavipes(Kollar), workers comprised the most abundant caste, with a 2-year average of 84.0% of the individuals. Larvae comprised 8.7%, nymphs 5.0%, adult soldiers 2.1%, and presoldiers and neotenic reproductives 0.1% each. Alates and eggs were found infrequently. Caste proportions changed significantly with season, apparently varying according to the reproductive cycle. Larvae are most abundant in the summer and least abundant in the winter. Soldiers were most abundant immediately preceding alate flights, and least abundant immediately thereafter. Nymphs showed two peaks of abundance (early fall and early summer). Workers were most abundant during the spring and summer months.