We have isolated from a laboratory strain of Culex pipiens in which abnormal larval mortality occurred a small icosahedral nonenveloped DNA virus sharing the main biological and biophysical properties of densoviruses (DNVs). Unlike DNVs isolated previously from Aedes species, i.e. the AaeDNV and the AalDNV (Afanasiev, B.N., Galyov, E. E., Buchatsky, L.P., Kozlov, Y.V., 1991. Nucleotide sequence and genomic organization of Aedes densonucleosis virus. Virology 185, 323-336; Boublik, Y., Jousset, F.-X., Bergoin, M., 1994. Complete nucleotide sequence and genomic organization of the Aedes albopictus Parvovirus (AaPV) pathogenic for Aedes aegypti larvae. Virology 200, 752-763), this mosquito DNV named CpDNV possesses a genome of 6 kb separately encapsidated in stoichiometric proportion as 'plus' and 'minus' strands. The lack of sequence homology between the CpDNV and the AalDNV genome and of antigenic cross-reactivity between their capsid polypeptides indicates that these two mosquito DNVs are phylogenetically distant. In contrast, the CpDNV appears to be related to the Junonia coenia densovirus (JcDNV) at both serological and genomic levels.