AbstractA method is presented allowing a clear distinction between bacterial viruses T3 and T7 by plating on selectively permissive host cells. The indicator strains areEscherichia colicells containing either clonedpifgenes (exclusively permissive for T3) or theEcoRV DNA restriction system (permissive only for T7): The efficiencies of plating of the two phages on these hosts differ by more than 8 orders of magnitude. This method was applied to reinvestigate the controversial question of mutual exclusion between T3 and T7. Under single‐burst conditions, about 50% of coinfected cells (permissive for both viruses) produced T3andT7 progeny while about 25% reproduced only T3 and about 25% only T7. The burst size of co‐infected cells was slightly reduced, compared to controls infected with only one virus type. Homologous exclusion among T3 phages was also not seen; rather, there was a gene dosage effect: T3‐encoded RNA polymerase activity as well as T3‐specific RNA synthesis increased proportionally to the multiplicity of infection (2.5–20 plaque‐forming