In a wider context of research on terrestrial isopods recently carried out on Mt. San Giuliano (Erice) in western Sicily, some specimens belonging to four species (Armadillidium decorum Brandt, 1833, Trichoniscus panormidensis n. sp., Philoscia affinis Verhoeff, 1908, and Porcellio siculoccidentalis Viglianisi, Lombardo & Caruso, 1992) have been collected showing a clear blue–purple coloration distributed on the body and the appendages. Scanning electron microscopy of tissues of A. decorum showed the presence of Iridoviridae, very similar to isopod iridescent virus (IIV), already known with other oniscidean species but not yet reported for Italy. The coloration is produced by characteristic paracrystalline arrays of virions inside the cells (patent infection). Viral particles are electron dense, showing a diameter of almost 0.12 μm. Dual-axis tomography performed on specimens of A. decorum confirmed the icosahedral structure showing a pentagonal or hexagonal section for the different levels observed. Molecular analysis on a portion of 254 bp of the MCP (major capsid protein) gene placed the virus as belonging to the IV31 group. BLAST analysis showed 98% of nucleotidic identity and 100% of aminoacidic identity with the MCPs of iridovirus from Armadillidium vulgare and from the beetle Popillia japonica.