The DNA synthesis in Bacillus amyloliquefaciens infected with wild‐type phage ϕ29 and with temperature‐sensitive mutants has been studied in the presence of the drug 6‐(p‐hydroxy‐phenylazo)‐uracil. The drug inhibits DNA synthesis in uninfected bacteria while it does not affect DNA synthesis and phage development in ϕ29‐infected bacteria. DNA synthesis was blocked by infection at the restrictive temperature with the mutants F99 and K132, representatives of the two early functions identified so far, as well as with mutant C17, which affects a late function. The remaining mutants, belonging to eight complementation groups, synthesized DNA at the restrictive temperature. The proportion of the DNA resistant to DNAase was different, depending on the mutant.