Larvae from six Drosophila species of the melanogaster subgroup were compared for both the hemolymph concentration of hemocytes and the ability to encapsulate the eggs of the parasitoid Asobara tabida (Hymenoptera; Braconidae). Results showed a high correlation between the parasitized hosts' concentration of circulating hemocytes and their aptitude to form a hemocytic capsule around the parasitic eggs. Two conditions seem to be required for the encapsulation of A. tabida eggs to succeed: one condition, which may relate to the recognition of the parasite by the host defense system, is the occurrence of a primary hemocytic response, which gives rise to the amplification of the hemocyte population; the other condition is the presence in the parasitized hosts of a hemocyte load large enough for the cellular capsule to be completed before the parasitic egg becomes protected by embedment within the host tissues. Since the concentration in hemocytes of the parasitized hosts is partially related to the concentration in hemocytes before parasitization, Drosophila species carrying a high hemocyte load could be better predisposed to resist A. tabida. Results are discussed in regard to the importance of a non-specific, quantitative character, such as the host hemocyte load, for the co-evolutionary immune interactions between A. tabida and its Drosophila hosts.