The optimum conditions for the expression of WSSV (= PRDV) rVP26 and rVP28 in E. coli cells were investigated, especially the IPTG concentration (0.001-1.0 mM) and inducement temperatures (25°C and 37°C). For both rVP26 and rVP28, the strongest intensity of expression was observed in medium containing 0.01 mM IPTG at 37°C. The growth of the transformed bacteria expressing rVP26 or rVP28 in media containing 0.01 mM IPTG was the almost same as that in medium without IPTG, but clearly delayed than that in the medium containing 1.0 mM IPTG. The productivity was increased 9-fold in rVP26 and 6-fold in rVP28 when the IPTG concentration was reduced from 1.0 mM to 0.01 mM. Next, we investigated the minimum doses of rVPs required to induce phylaxis of kuruma shrimp Penaeus japonicus against WSD (= PAV) by an experimental challenge. The results show that at least 10 μg/g shrimp/day of rVPs should be orally administered to kuruma shrimp for 15 days in order to induce effective phylaxis.