The susceptibility of carp Cyprinus carpio to koi herpesvirus (KHV) was examined by experimental infections. Larvae of two strains of common carp were exposed to KHV by immersing the fish in water containing the virus at a dose of 101.6TCID50/mL measured using KF-1 cells. No mortality by KHV disease was observed among any larvae (average TL: 7.5 and 8.7 mm). On the other hand, high mortalities (69% and 100%) were observed in common carp juveniles (TL: 13.8 and 29.2 mm). Koi carp larvae (TL: 6.9 mm) were challenged in the same experimental conditions as described above, in which no larva died. Two months later, the same fish (TL: 48.2 mm) were exposed to the virus again. This time, all fish died of KHV. These results suggest that carp larvae are not susceptible to KHV, and that they become susceptible as they grow up.