The secondary structure of the autoregulatory mRNA binding site of Escherichia coli ribosomal protein L1 has been studied using enzymatic methods. The control region of the E. coli L11 operon was cloned into a vector under control of the Salmonella phage SP6 promoter, and RNA transcribed using SP6 RNA polymerase. The secondary structure of this RNA was probed using structure-specific nucleases, and by comparison of the data with computer predictions of RNA folding, secondary structural features were deduced. The proposed model is consistent with elements of some previously proposed models, but differs in other features. Finally, secondary structure information was obtained from two mutant mRNAs and the structural features correlated with observed phenotypes of the mutants.