The phenomenon of exclusion in RNA phages of E. coli was investigated with four phages, β, I, Y, and Z, which could be distinguished from one another by their serological characters and plaque types. When E. coli cells were infected simultaneously with two serologically related phages, no exclusion was observed, but the exclusion was apparent when the cells were infected with two serologically unrelated phages. A phage having been judged to be highly "virulent" by the simultaneous infection method was excluded by a weakly "virulent" phage when tested by the superinfection method. The results obtained were compared with the exclusion phenomena in E, coli DNA phages.