The structure formed by a chimeric alpha-beta-oligonucleotide containing the 3’-end alpha-sequence complementary to the 5’-end beta-sequence is described. Chemical strand cross-linking was used to probe the structure of this oligonucleotide. The terminal sequences form a stable parallel duplex, and the internal sequence of the oligonucleotide forms a circle-like loop. This new molecule is resistant to nucleolytic degradation and also promotes efficient RNase H-mediated cleavage of a complementary ribooligonucleotide. Oligonucleotides with this structure therefore should prove useful in the control of gene expression.