Abstract
Proteus mirabilis strains 13 and 5006 on rare occasions liberate temperate phages, 13M and 5006M, which may reinfect parent strains. These prophages are not inducible but the probability of phage liberation may be increased by growth in nitrosoguanidine. P. mirabilis strain 34 liberates a phage, 34, spontaneously. Strain 34 is doubly lysogenic for this phage. Phages 13M, 5006M and 34 are serologically identical and convert their hosts. The cryptic lysogeny of these strains may be due to the mode of integration of the prophages in the bacterial chromosome.