Summary. -We investigated the effect of growth phase of suspension culture of insect Sf9 cells on cell cycle phase distribution, cell viability and Autographa californica multiple nucleopolyhedrovirus (AcMNPV) production. The cell culture showed a maximum cell viability and potential to replicate the virus at the peak of G 1 phase cells in the culture, while the minimum cell viability coincided with the peak of G 2 /M phase cells. These results indicate that the G 1 phase plays a substantial role in the ability of cells to replicate the baculovirus and may help to develop a baculovirus infection dynamics model and control the expression of foreign genes.Keywords: cell cycle; baculovirus; viability; flow cytometry; infection E-mail: yhz@mail.wit.edu.cn; phone: +86-13871114386. Abbreviations: AcMNPV = Autographa californica multiple nucleopolyhedrovirus; BEVS = baculovirus expression vector system; FCM = flow cytometry; NOV = non-occluded virus; OV = occluded virus; TOI = time of infection