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Phages of Lactobaciilus casei (PLC) isolated from plant drainage were classified and characterized. The results are as follows : 1. On the basis of host range pattern, phages could be divided into 2 groups (PLC-B and PLC-C). PLC-B group phages could be further divided into 5 sub-groups (B₁, B₂, B₃, B₄and B_5). Although PLC-C group phages had the same host range, they could be also divided into 2 sub-groups (C₁and C₂) by morphological type. 2. It was B₃group phages that represented a major proportion (44.4%) of phages tested. However, B₁group phages were shown to have the widest host range. 3. Electron micrographs revealed that the phages fell into three different morphological types. B₁, B₂and B₃group phages had a hexagonal head (52 nm in diameter) and a sheathless noncontractile tail (245 nm in length). B₄and C₂group phages had a hexagonal head (56 nm) and a short flexible tail (169nm) having no sheath. B_5 and C₁group phages were shown to have a hexagonal head (81 nm) and a contractile tail (140 nm) having a sheath, a base plate and tail fibers. 4. The inactivation of the phages by antisera indicated that serological relationships correlated completely with morphological types. 5. B₁, C₁and C₂group phages produced a large (1,2 mm in diameter) plaque with a clear ring. The morphology of plaques of B₃and B_5 group phages was the same as those produced by the above, but the average plaque sizes for B₃and B_5 were 0.8 mm and 0.5 mm, respectively. B₂and B₄group phages produced a small (0.5 mm) turbid plaque with an irregular edge. 6. The latent period and the average burst size of B₁and B₃group phages were 90 min and 100, respectively. These phages required calcium ions for their multiplication. 7. B₃roup phages could not be absorbed to R-variant KC₁.
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