Summary:The present report gives the results of the effect of 10 bacteriophages ofSerratia marcescenson 107 strains of the genusSerratia. The phage S103 was the most effective (80% strains lyzed), and the phage PK the least (12% strains lyzed). Only 13 strains were resistant to all the phages studied. The majority of the strains of the genus studied could be allocated to one of seven groups on the basis of their sensitivity to the phages.