Semi-random libraries of DNA 6mers and RNase H digestion were applied to search for sites accessible to hybridization on the genomic and antigenomic HDV ribozymes and their 3' truncated derivatives. An approach was proposed to correlate the cleavage sites and most likely sequences of oligomers, members of the oligonucleotide libraries, which were engaged in the formation of RNA-DNA hybrids. The predicted positions of oligomers hybridizing to the genomic ribozyme were compared with the fold of polynucleotide chain in the ribozyme crystal structure. The data exemplified the crucial role of target RNA structural features in the binding of antisense oligonucleotides. It turned out that cleavages were induced if the bound oligomer could adapt an ordered helical conformation even when it required partial penetration of an adjacent double-stranded region. The major features of RNA structure disfavoring hybridization and/or RNase H hydrolysis were sharp turns of the polynucleotide chain and breaks in stacking interactions of bases. Based on the predicted positions of oligomers hybridizing to the antigenomic ribozyme we chose and synthesized four antisense DNA 6mers which were shown to direct hydrolysis in the desired, earlier predicted regions of the molecule.