Technical Abstract: We present morphological and molecular evidence that a new mosquito virus, Culex nigripalpus baculovirus, is a member of the family Baculoviridae yet distinct from the general Nucleopolyhedrovirus and Granulovirus. Development is restricted to gastric caeca and posterior stomach of midgut epithelial cells. The globular occlusion bodies are not enveloped, measure earound 400 nm in diameter and occur exclusively in nuclei of infected cells. Each occlusion body typically contains four, sometimes up to eight, virions. The developmental sequence involves two virion phenotypes: an occluded form (ODV) that initiates infection in the midgut epithelial cells, and a budded form (BV) that spreads the infection in the midgut. Each ODV contains one rod-shaped enveloped nucleocapsid (40x200mn). The double-stranded DNA genome is approximately 105-110 kbp with an estimated GC content of 52 percent. We have sequenced over one third of the genome and detected 79 putative ORFs of 50 amino acids or more including 8 genes considered to be unique to baculoviruses. Phylogenetic analysis of the amino acid sequences of dnapol and p74 suggested that C. nigripalpus is a member of a new genus, Ventriculovirus n.g., within the family Baculoviridae.