On the basis of the length of tail, two morphological types of S. aureus bacteriophages of the serogroup B converting fibrinolysine were shown. The properties of the 22 S. aureus bacteriophages of the serologic group B were investigated. All of the investigated bacteriophages were classified as the morphological group II of the family Styloviridae on the basis of electron microscope analysis. The size of capsids of the examined bacteriophages was 53.85 +/- 1.05 nm and the tail length varied from 135.4 nm to 170.5 nm. All of them had the tail terminated in the basal plate. The lytic properties of the investigated bacteriophages were not identical. 7 of them showed features of lytic group I and 15 of lytic group III. The members of lytic group III had apparently a longer tail than bacteriophages of lytic group I.