A very low incidence (< 0.01%) of a blue iridovirus (IV) was found in larvae of the Japanese beetle, Popillia japonica Newman, that were sampled over a two year period on Terceira Island (Azores, Portugal). In the most heavily infected larvae, a deep blue iridescence was observed, particularly in the fat body. Transmission electron microscopy revealed the characteristic crystalline arrays of the hexagonal virus particles in the cytoplasm of fat body cells, tracheal matrix, muscle, hypodermis and blood cells. Crystals of the virus particles were also observed freely circulating in the hemolymph. The average diameter of negatively stained purified virus particles was 157 nm. Similarities and differences with other IVs found in the Scarabaeidae are discussed. Considering the broad host range of some of the iridescent viruses, the relatively recent invasion of Terceira by P. japonica, and the rarity of the virus in the beetle, it is probable that the infection was the result of transmission from another species of soil-inhabiting arthropod. Its value as a potential biological control agent of P. japonica is negligible.