Streptococcus thermophilus autolytic strains are characterized by a typical bell-shaped growth curve when grown under appropriate conditions. The cellular mechanisms involved in the triggering of lysis and the bacteriolytic activities of these strains were investigated in this study. Lactose depletion and organic solvents (ethanol, methanol, and chloroform) were shown to trigger a premature and immediate lysis of M17 exponentially growing cells. These factors and compounds are suspected to act by altering the cell envelope properties, causing either the permeabilization (organic solvents) or the depolarization (lactose depletion) of the cytoplasmic membrane. The autolytic character was shown to be associated with lysogeny. Phage particles, most of which were defective, were observed in the culture supernatants after both mitomycin C-induced and spontaneous lysis. By renaturing sodium dodecyl sulfate-polyacrylamide gel electrophoresis, a bacteriolytic activity was detected at 31 kDa exclusively in the autolytic strains. This enzyme was detected during both growth and spontaneous lysis with the same intensity. We have shown that it was prophage encoded and homologous to the endolysin Lyt51 of the streptococcal temperate bacteriophage phi 01205 (M. Sheehan, E, Stanley, G. F. Fitzgerald, and D. van Sinderen, Appl. Environ. Microbiol. 65:569-577, 1999). It appears from our results that the autolytic properties are conferred to the S. thermophilus strains by a leaky prophage but do not result from massive prophage induction. More specifically, we propose that phagic genes are constitutively expressed in almost all the cells at a low and nonlethal level and that lysis is controlled and achieved by the prophage-encoded lysis proteins.