The ability of conditional lethal mutants of phage ΦX174 to induce host cell lysis during infection under restrictive conditions has been studied. We have found amber (am) and temperature-sensitive (ts) mutants which present a variety of alterations in the normal lytic process. In particular, there is a class of am mutants which do not produce cell lysis but otherwise replicate normally in the restrictive host. These mutants constitute a single complementation group. The existence of these mutants implicates a phage-coded protein in the lytic process. This protein is not an essential structural component of the phage, since normal phage particles are produced in the absence of lysis