The multiplication of CIV was analysed by markers such as iridescence, cytoplasmic inclusion, and infectivity titer of the hemolymph or homogenate of insect. 1. Experimental hosts of CIV found by per os inoculation and intrahemocoelic injection were as follows: Lepidoptera: 65 species (26 families); Coleoptera: 8 species (7 families); Hymenoptera: 2 species (2 families); Diptera: 2 species (2 families); Orthoptera: 2 species (1 family). 2. Apparent increase of the infectivity was shown in ID_50-time curve in the hemolymph of Atractomorpha bedeli (Orthoptera: Acrididae). In Acrida turrita (Acrididae), iridescence was observed in CIV-injected nymph. These results are the first evidence of the multiplication of the iridescent virus in orthopteran insects. 3. In general, the appearance of iridescence was accompanied with the increase of CIV-infectivity. However, no iridescence could be observed in some cases, although the increase of infectivity in hemolymph or homogenates of whole bodies were observed. Accordingly, the infectivity titer should be considered to be the most important marker in CIV-infection. 4. The susceptibility of insects to CIV remarkably increased during metamorphosis from larvae to pupae in the quantitative experiments on the multiplication of CIV using lepidopterous insects such as Bombyx mori (Bombycidae) and Mamestra brassicae (Noctuidae). 5. Serial passages of CIV through B. mori and Philosamia cynthia pryeri (Saturniidae) were successfully done. 6. The infectivity of CIV to larvae of the silkworm, B. mori remarkably increased after serial passages through the larvae of the same host insect. 7. Adults of B. mori emerged from CIV-infected pupae copulated and oviposited normally, although their hemolymph showed high infectivity. CIV-infectivity of meconium from these newly emerged adults showed also relatively high level of titer. 8. Relatively many species from Lepidoptera, Orthoptera, Bllattaria, lsoptera, Neuroptera, Hemiptera, and Dermaptera were not susceptible to CIV, and defense mechanisms in these insects should be further analysed.