Résumé : Seven virulent bacteriophages of Streptococcus thermophilus were characterized at the molecular level and classified into 2 subgroups (A and B) by DNA/DNA hybridization experiments and analysis of their structural proteins. Two representatives of subgroups A and B were compared to 3 representatives of Neve's subgroups I, II and III (Neve et al, 1989) by Southern blot experiments. These isometric-headed phages possess a double-stranded DNA genome varying between 30-44 kilobase (kb) pairs. Subgroup A is composed of 3 phages (F{cyrillic} 57 as representative) with similar structural proteins as determined by sodium dodecyl sulfate-polyacrylamide gel (SDS-PAGE) electrophoresis (estimated molecular weights of 31 000 and 27 500 for phage F{cyrillic} 57 and 32 000 and 27 000 for the 2 others). A common structural protein of 43 000 was found for phages of subgroup B. Phages F{cyrillic} 57 (subgroup A) and a10/J9 or PO (Neve's subgroups I or II, respectively) belonged to the same subgroup as determined by DNA/DNA hybridization experiments. Partial DNA homology was detected among all the phages tested except for phage F{cyrillic} ST27 of AW Jarvis. Phage-host interactions were also investigated by cross-propagation of the 7 studied phages on different indicator strains. A complete lack of correlation existed between the DNA homology grouping of the phages and their host range. Various restriction-modification systems were detected in some of the Streptococcus thermophilus strains. © 1990.