Spodoptera exigua nucleopolyhedrovirus (SpeiNPV) isolated in Shiga Prefecture, Japan, was bioassayed with all five stages of the beet armyworm to determine lethal doses and survival times at three rearing temperatures: 20°C, 25°C, and 30°C. Rearing temperature did not significantly affect the susceptibility of S. exigua larvae to SpeiNPV. The median lethal dose (LD50) per insect for neonate, second-, third-, fourth-, and fifth-stage larvae was 2.5, 11.2, 5.5, 32.4, and 181.8 polyhedral occlusion bodies (POBs), respectively. These data demonstrate that this SpeiNPV isolate is extremely infectious, even to later stages of S. exigua larvae. The LD50 values corrected per unit larval body weight tended to decrease with larval age, from about 60 POBs for neonates to just a few POBs for each of the last three stages. This pattern is atypical for NPV-lepidopteran host insect systems, emphasizing the high infectivity of this NPV isolate. Rearing temperature and larval stage at the time of viral treatment significantly affected survival times, while viral dosage did not. Survival time decreased with increased rearing temperature, and increased with larval stage at the time of treatment. Median survival time (ST50) was between 3 to 14 d, depending on the rearing temperature and larval stage at the time of viral treatment.