The crystal structure of a genomic hepatitis delta virus (HDV) ribozyme 3' cleavage product predicts the existence of a 2 bp duplex, P1.1, that had not been previously identified in the HDV ribozymes. P1.1 consists of two canonical C-G base pairs stacked beneath the G.U wobble pair at the cleavage site and would appear to pull together critical structural elements of the ribozyme. P1.1 is the second stem of a second pseudoknot in the ribozyme, making the overall fold of the ribozyme a nested double pseudoknot. Sequence comparison suggests the potential for P1.1 and a similar fold in the antigenomic ribozyme. In this study, the base pairing requirements of P1.1 for cleavage activity were tested in both the genomic and antigenomic HDV ribozymes by mutagenesis. In both sequences, cleavage activity was severely reduced when mismatches were introduced into P1.1, but restored when alternative base pairing combinations were incorporated. Thus, P1.1 is an essential structural element required for cleavage of both the genomic and antigenomic HDV ribozymes and the model for the antigenomic ribozyme secondary structure should also be modified to include P1.1.