In vivo entry of Bombyx mori cypovirus 1 (BmCPV-1) into silkworm midgut cells was studied by elec tron microscopy. Virions were observed adhering to the plasma membrane of microvilli of the columnar cells, embedding in the membrane, and settling themselves intact inside the microvilli. These behaviors suggested that intact BmCPV-1 virions enter columnar cells by means of direct penetration through the cell membrane. In addition, goblet cells, muscle cells and the hemocoele were also involved in early events of the virus infection. However, no replication of the virus had ever been detected in these invaded cells except for columnar cells.