Genetic variation in susceptibility to pathogens is a central concern both to medicine and agriculture and to the evolution of animals. Here, we have investigated the link between Such natural genetic variation and the immune response in wild-type Drosophila melanogaster, a major model organism for immunological research. We found that within nine wild-type strains, different Drosophila genotypes show wide-ranging variation in their ability to survive infection from the pathogenic bacteria Listeria monocytogenes. Canton-S, a resistant strain, showed increased capacity to induce stronger innate immune activities (antimicrobial peptides (AMI's), phenol oxidase activity, and phagocytosis) compared to the Susceptible strain (white) at early time points during bacterial infection. Moreover, PGRP-LE-induced innate immune activation immediately after infection greatly improves Survival of the susceptible strain strongly Suggesting a mechanism behind the natural genetic variation of these two strains. Taken together we provide the first experimental evidence to suggest that differences in innate immune activity at early time points during infection likely mediates infection susceptibility in Drosophila. (C) 2008 Elsevier Inc. All rights reserved.