The relative activity of three concentrations of Spodoptera littoralis nuclear polyhedrosis virus (SLNPV) against Ceratitis capitata were determined by (a) submerging two egg age groups, freshly laid eggs (0–2.5 hr) and more older ones (0–24 hr) in viral concentrations, and (b) feeding newly hatched larvae on viral contaminated media. Pupations that resulted from all viral treatments were decreased with increase of virus dose. Hatchability showed the same relation, but hatchability of treated old eggs were not significantly influenced by increases in viral concentration. Adult emergence was significantly reduced due to viral treatments to eggs, however, it was increased in the case of feeding the larvae on contaminated diet. The most hypothetical probability of this deleterious effect may be attributed to an activation of an indigenous virus already present in C. capitata. More research is required to understand the basis for the deleterious effect of SLNPV on the fruit fly.