The bacteriophages studied in the present investigation were obtained directly from the soil by means of a strain of pea Rhizobium P1l. The crude culture of the phage, which was designated S2P1i, has been the subject of two previous papers (Kleczkowska, 1945a (Kleczkowska, , 1945b. This phage, when supplied to different strains of Rhizobium, produced plaques on susceptible bacteria that differed in mean numbers and size according to the bacterial strain used. This suggested that phage S2P11

more »

... t represent a mixture of different strains of phage. This paper describes several such strains isolated from phage S2P11 by means of successive passages through different susceptible bacterial strains and by cultivation from a single plaque.