Superinfection by phage T4 of cells infected by the ribonucleic acid (RNA) phage f2 results in inhibition of further f2 production. Experiments using rifampin show that the exclusion of f2 requires T4 gene function soon after T4 infection. By using a sensitive new peptide-mapping procedure to identify f2 coat protein in infected cells, we show that synthesis of the f2 coat occurs at a reduced level until 4 min after T4 superinfection and then ceases abruptly. Within 4 min after T4 superinfection, there are also several changes in f2 RNA metabolism, all of which require T4 gene function: preexisting f2 replicative intermediate RNA and f2 single-stranded RNA are degraded to small but still acid-precipitable fragments, and most f2-specific RNA is released from polyribosomes. We favor the hypothesis that T4 induces the synthesis of a specific endoribonuclease which degrades f2 RNA and that the inhibition of f2 protein synthesis may be a consequence of this degradation, rather than a direct effect of T4 upon translation.