Cytoplasmic polyhedrosis virus (cypovirus, CPV) isolates from larvae of saturniidae silkworms Antheraea mylitta, Antheraea proylei, and Antheraea assamensis were obtained and characterized. Scanning electron microscopic (SEM) studies revealed the presence of hexahedral (rhombic dodecahedron) polyhedral bodies of various size (1-4 microns) in individual hosts while transmission electron microscopic (TEM) studies of isolated virions showed their uniform (icosahedral) shape and size (approximately 55 nm). Genomes of these isolates were obtained from purified polyhedral bodies and analyzed by agarose and polyacrylamide gel electrophoresis. Eleven bands ranging from about 350 bp to 3.9 kb (total of 25.35 kb) were detected. These bands were completely degraded after treatment of the genome preparations with RNase A at low salt but not at high salt concentration. The banding pattern of the genomes remained unaffected by treatment of the respective preparations with DNase I, indicating that the genomes were composed of 11 double-stranded (ds) RNA segments. The RNA banding patterns of the CPV isolates from the saturniidae silkworms were similar to each other. These characteristics were similar to that of Antheraea mylitta cypovirus 4 (AmCPV-4) but different from that of Bombyx mori cypovirus 1 (BmCPV-1). In comparison to AmCPV-4 the isolates contained an additional, eleventh RNA segment and the size of the RNA segments was also different. Dot blot hybridization showed hybridization of the cloned 9th and 10th segment cDNAs from the A. mylitta CPV (AmCPV) isolate with RNAs from the A. proylei and A. assamensis CPV (ApCPV, AaCPV) isolates but not with RNA from BmCPV-1. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS-PAGE) of polyhedral proteins from the AmCPV isolate showed the presence of 3 major and 5 minor bands of M(r) from 29.4 K to 163 K, but BmCPV-1 polyhedral bodies showed different protein banding pattern. Two heterologous cell lines infected with the AmCPV isolate showed very slow virus growth with the accumulation of polyhedral bodies in the cytoplasm of infected cells indicating a restricted host range of this CPV isolate.