FOr many years it has been a matter of common knowledge that bacteriophages can be easily isolated from water supplies. They were first demonstrated in water by Dumas in 1920, and many authors have since confirmed his observation (Arloing & Chavanne, 1925; Manoliu & Costin, 1925; Arloing & Sempé, 1926; Bilouet, 1926; d'Herelle, 1926; Monteiro, 1926; Bujanowski, 1929; Gildemeister & Watanabe, 1931; Stewart & Ghosal, 1931–1932; Schuurman & Schuurman-ten Bokkel Huinink, 1934; Dienert, 1934; Vagedes & Gildemeister, 1934; Panayotatou, 1935; Nick, 1936). These workers have recorded the presence of phages lysing dysentery, coliform, typhoid, paratyphoid, and, sometimes, cholera organisms in a wide variety of water supplies: e.g. lake, river, stream, canal and tap waters; phages have also been detected in sea water close to the shore.