The abundant mRNAs used as templates for synthesis of filamentous phage f1 proteins are a combination of primary transcripts and 3' products of processing. The processing steps are mediated by host endoribonucleases. One of the enzymes implicated in f1 mRNA processing is RNase E, the only endonuclease thus far shown to have a global role in mRNA decay. By establishing the temperature-sensitive phenotypes of RNase E mutants and then inducing a transcription unit bearing cloned f1 processing sites, we show that RNase E is required for production of at least three of the processed RNAs. Using in vivo processing assays, we also test directly the regions implicated genetically in previous work to contain the processing sites. The sites function as discrete domains in a number of transcription units, show little influence of translation, but appear to have increased activity at the 5' terminus of an mRNA. From their functional properties, we suggest that the known processing sites from phage f1 that are dependent on RNase E may be representative of relatively late steps in rne-dependent cleavage pathways.