ABStrACt Survival and caste differentiation were observed under controlled con-ditions in orphaned experimental colonies of the subterranean termites Reticulitermes grassei and R. santonensis. Worker colonies had different sizes (30, 50, 100, 200 and 300); after 12 and 32 months the differentiation of colony members in other castes was observed. twelve months after orphan-ing, 80% of the colonies had survived. For the two species, a mean number of one soldier was observed in 7 colonies and between 1 and 3 nymphs were present in 18 colonies, whatever the initial number of workers. In 53% of the surviving colonies, the differentiation of secondary reproductives occurred and they produced viable offspring. The external morphology of R. grassei male reproductives did not differ significantly from those of workers or nymphs. Thirty-two months after orphaning, colonies with an initial number of 30 workers were comprised of secondary reproductives and their offspring. In termite species, caste differentiation pathways and thus the caste system are highly flexible. Therefore, our results show that a small number of subterra-nean termites could establish a new colony within few years and thus invade a new habitat, for example in urban areas.