The temperate phages of Xanthomonas citri are completely inactivated by chloroform treatment, whereas the virulent phages are quite stable to the treatment. About 80 percent (but not all) of an extensive series of X. citri isolates is lysed by phage Cp1. Cp2, or both. Among 136 isolates of 43 Xanthomonas nomenspecies, only 9 isolates belonging to 7 nomenspecies [X. begoniae (XB8, XB111), X. blepharidis (XB115), X. cucurbitae (XC6), X. phaseoli (XP4, Xph), X. phaseoli var. sojense (XP21), X. marantae (XM126), and X. translucens f. sp. phleum-pratense (XT103)] showed susceptibility to any of the X. citri phages. One of the Stolp-Starr phages, ph25, showed lytic reactions on almost all isolates of X. citri, as well as on many other Xanthomonas nomenspecies. From the cross-susceptibility tests between Xanthomonas bacteria and various phages from different sources, it was concluded that, in general, the Xanthomonas phages which form tiny plaques of pinhead size could frequently be found in soil, and that they showed very broad host ranges, which extended not only over the different isolates of a homologous nomenspecies but also over strains of many heterologous nomenspecies. In contrast, the Xanthomonas phages which were isolated from diseased plant tissues usually showed very limited host ranges.