Autographa californica nuclear polyhedrosis virus (AcNPV) is a large, double-stranded DNA virus that infects lepidopteran insects. In the course of infection two different forms of the virus are produced; one that serves to spread the infection from insect to insect and another that spreads the infection within the insect from cell to cell. The latter form matures by budding from the plasma membrane of infected cells. We have found that cytochalasins B and D prevent the production of infectious budded AcNPV, not by inhibiting budding but by inhibiting the synthesis of complete virions. In the presence of cytochalasins B and D, particles lacking nucleocapsids bud from the plasma membrane of AcNPV infected cells, implicating microfilaments in the synthesis, transport and/or assembly of critical virus components.