The RNA bacteriophage R23 inhibits the synthesis of bacterial RNA and protein to a greater extent than phages f2, Qβ, R34, or R40. Various aspects of the process of infection of Escherichia coli by these phages have been examined to assess their possible role in determining the extent of viral RNA synthesis and inhibition of the host. There was little difference between R23, R34, R40, and f2 in the rate of adsorption and "eclipse of infectivity," time of appearance of the first intracellular phage, and overall efficiency of infection at multiplicities over 5 phage/cell. But the phage yield of R23, in terms of plaque-forming units and total particles, was higher than that of R34, R40, or f2. R23 also had the longest life cycle with a relatively prolonged latent period. Qβ appeared to infect cells less efficiently under the experimental conditions used; this probably accounted for its lesser detectable effect on host metabolism. In successfully infected cells, the particle yield of Qβ may be comparable to that of R23 but the yield of viable phage was less.