Acanthamoeba polyphaga mimivirus (APMV) is a species of DNA virus in the Mimivirus genus of the Mimiviridae family. It infects the amoeba, Acanthamoeba polyphaga. Its non-enveloped icosahedral capsid is 400 nm in diameter, with protein filaments of 100 nm projecting from its surface. The APMV genome is a linear, double-stranded DNA molecule of around 1.2 megabases, encoding around 979 genes. This is comparable to the genome of some small bacteria. It encodes several proteins that had not been previously discovered in viruses, including aminoacyl tRNA synthetases. APMV is itself parasitised by the Sputnik virophage (arrowed in micrograph).
APMV is as large as some small species of bacteria, such as Rickettsia conorii and Tropheryma whipplei. When it was first discovered in 1992, it was thought to be a bacterium, and named Bradfordcoccus. APMV was not shown to be a virus until 2003, when it was the largest virus then discovered. It has since been overtaken by Megavirus chilensis, Pandoravirus and Pithovirus, all of which also infect amoebae. These and other large and complex DNA viruses are now grouped in Nucleocytoviricota, also termed nucleocytoplasmic large DNA viruses.