Within the icosahedral protein outer shell of bacteriophage T7, a 40-kbp DNA genome occupies a cavity also occupied by a protein cylinder that projects into the DNA from the outer shell. However, neither the internal cylinder nor separately resolved DNA segments are revealed in the conventional negatively stained specimens of intact bacteriophage T7. In the present study, a procedure of negative staining is used that reveals both internal proteins and separately resolved segments of packaged DNA during electron microscopy of intact particles of a hybrid T7 bacteriophage; the hybrid is genetically T7, except for a tail fiber gene that has a segment from the T7-related bacteriophage, T3. The negatively stained packaged DNA segments of this hybrid bacteriophage are found to be wrapped around the axis of the internal cylinder. To obtain additional information about the conformation of packaged T7 DNA electron microscopy is performed of negatively stained capsids that are incompletely filled with DNA (ipDNA-capsids); a procedure is described for improved isolation of ipDNA-capsids from lysates of hybrid bacteriophage T7-infected cells. The packaged DNA segments of ipDNA-capsids are found not to be wrapped around any axis. Images of ipDNA-capsids are explained by the hypothesis that DNA does not achieve its wrapped condition until the capsid is more than 40% full of DNA. Wrapping via folding is, therefore, proposed to explain the images of DNA packaged in bacteriophage T7. (C) 1997 Academic Press.