The Escherichia coli (E. coli) ribonuclease E protein (RNase E) is implicated in the degradation and processing of a large fraction of RNAs in the cell. To understand RNase E function in greater detail, we developed an efficient selection method for identifying nonfunctional RNase E mutants. A subset of the mutants was found to display a dominant-negative phenotype, interfering with wild-type RNase E function. Unexpectedly, each of these mutants contained a large truncation within the carboxy terminus of RNase E. In contrast, no point mutants that conferred a dominant-negative phenotype were found. We show that a representative dominant-negative mutant can form mixed multimers with RNase E and propose a model to explain how these mutants can block wild-type RNase E function in vivo.