Data previously obtained indicate that, during assembly of the related bacteriophages T7 and T3, a DNA-free procapsid (capsid I) is produced and that subsequently capsid I: (1) binds to a longer than mature (concatemeric) DNA and then becomes structurally altered to a particle isolated as a capsid (capsid II) physically resembling the mature bacteriophage capsid more than the procapsid (initiation phase of packaging), (2) draws DNA to its interior (entry phase of packaging), (3) participates in cutting the concatemeric DNA to mature size. It was found that, after infection of Escherichia coli mutant tsnB (selected for a deficiency in plating T7; M. Chamberlin [1974], J. Virol. 14, 509-516), T7 and T3 capsid I is assembled at a rate not significantly different from its rate of assembly in the wild-type host. However, the conversion of capsid I to capsid II was slowed in E. coli tsnB, suggesting that the tsnB mutation interferes with the initiation of DNA packaging. Although some T3 and T7 DNA enters capsids and is cut to mature size in the tsnB mutant, the data further suggest that the entry rate of DNA into capsid II is lower in the tsnB mutant than it is in an unaltered host. T7 capsid II-concatemeric DNA complexes accumulate during infection of the tsnB mutant. These observations suggest that use of the tsnB mutant as a host will simplify studies of bacteriophage T7 and T3 DNA packaging.