This comn~unication describes a modification of well-liuown techniques for the isolation of bacteriophages, fiilethods usually depend on the culturing of given host bacteria with source material, e.g. soil (3), sewage (2), or plant residues (5), in which the presence of phage is suspected. One expects phage particles present to multiply a t the expense of the added host cells so that single plaque isolates may readily be obtained. Such mass-selection methods are appropriate when the aim is to obtain a virulent phage for diagnostic (5) purposes. There exists the possibility that other phages might not be detected if they were, for example, of low virulence, slow to absorb to the host, small burst size, long latent period, or lysogenizing rather than lytic. We tried to detect temperate phage in samples of arable soils by a modification of the massselection technique involving (a) limited opportunities for multiplication of infected bacteria or of already-present phages in the enrichment culture and (b) antibiotic-resistant host bacteria together with appropriate antibiotics to reduce microbial contamination which might otherwise obscure rare plaques in soft agar layer plates (1). One strain each of Rhizobiz~m trifolii (RT2), R. meliloti (RM6), R. leguminosarz~m (RLS), R. plznseoli (RP2), and Xantlzomonas phaseoli (XPS) was used. Medium Y (6) was emploj~ed for the growth of Rhizobiz~wz and antibiotic medium 3 (Difco) for Xantlzomonas. Mutants resistant to 2000 pg/ml of streptomycin (calcium chloride conlplex, fiiIerk) were obtained by singlestep selection on appropriate solid media containing streptomycin. Soil samples were obtained from the legume-crop nursery plots, Central Experimental Farm, Ottawa, and stored in screw-cap jars. In experiments, about 50 g of field soil were enriched with a total of approximately 5 X lo9 bacteria, of one of the mutant strains mentioned above, suspended in 5 ml of antibioticcontaining liquid medium (500 pg/ml streptomycin). This mixture of soil with host bacteria, hereafter referred to as the enriched soil, was incubated for 16 hours to allow absorption and limited multiplication of soil-borne phages. Since we have never found phage titers greater than 105/g, and often much less, after enrichment, we assume that the number of cycles of phage infection, liberation, and reinfection of phage-sensitive cells is probably small. After 16 hours a t room temperature (approximately 2S0 C) 2 g of the enriched soil were suspended in 10 ml liquid medium for 2 hours. Particles of soil and vegetable debris were then sedimented by low-speed centrifugation and the supernatant tested for the presence of phage by plating 0.1-ml and 0.5-ml samples in soft agar layer (1) together with a culture of the appropriate