The mosquito iridescent viruses (MIVs) are large icosahedral DNA viruses that replicate and assemble in the cytoplasm of the host. Paracrystalline arrangements of virions that accumulate in the cytoplasm produce an iridescent color that is symptomatic of acute infections. In August 2010 we found larvae of Culex pipiens with these symptoms in suburban ditches around La Plata city, Argentina. Electron microscope studies, PCR amplification of the Protein (MCP) gene arrangement, DNA sequencing and phylogenetic analysis were carried out.