This study was undertaken to establish a typing scheme for Listeria monocytogenes. A total of 823 strains, isolated in France from 1958 to 1978, were studied; 69.4% of these belonged to serotype 4. Using a set of 20 phages, the lytic activity, frequency and specific character of these phages were estimated and phage typing carried out. We were able to define a phage typing system for L. monocytogenes using 12 principal phages and 3 secondary phages. In order to discriminate phages types within serotype 1, the phages 1, 3, 4, 5, 6 and 7 were used, and for phage types within serotype 4, the phages 10, 11, 14, 15, 16 and 17. Secondary phages 8, 9 and 20 had a weak lytic activity, but their specificity was very high. The lytic patterns obtained with the phages 9 and 20 were restricted to strains of serotype 5. We established a complete correlation between the lytic pattern of phages 1 to 8 and serotype 1, just as there was a correlation between the lytic pattern of phages 10 to 19 and serotype 4. Using the set of 20 phages we were able to type 645 of 823 strains of L. monocytogenes. Thus we were able to type 78.4% of all the strains examined. This percentage was very different according to the serotype of the strains tested: 88% of strains of serotype 4 and 57% of strains of serotype 1. With this set of phages 552 of 645 typable strains could be subdivided into 8 principal phagetypes: 3 types within the serotype 1 and 5 others in serotype 4. This phage typing system can be used in some epidemiological situations, in taxonomic investigations or as bacterial markers.