The hymenopteran Macrocentrus cingulum is a polyembryonic endoparasitoid that uses larvae of the lepidopteran Ostrinia furnacalis as one of its host insects. Previous studies indicated that although this parasitoid does not transmit polydnaviruses when it lays its eggs, a layer of fibrous tissue on the surface of the eggs helps them to avoid being encapsulated by the immune system of the host insect. However, as eggs of M. cingulum that are laid into late instar larvae of O. furnacalis often do not survive, there is a tendency for the adults to lay their eggs in earlier instar larvae. We studied the amounts of encapsulation and melanization around beads of DEAE-Sephadex A-25 injected into the haemoceol of fourth and fifth larval instars of O. furnacalis. The beads were injected to simulate the presence of eggs of the parasitoid M. cingulum. We found that the levels of encapsulation and melanization around the beads increased with the age of the O. furnacalis larvae. Likewise, the total counts of the haemocytes circulating within the haemolymph increased with the age of the O. furnacalis larvae and were correlated negatively with the percentage of larvae parasitized by M. cingulum. It appears that young O. furnacalis possess a weak cellular defence, and as a result are more susceptible to being parasitized. Hence, the correlation between the levels of encapsulation and the age of the host insect appears simply to reflect passive evasion. none