Aptamers are often prone to nuclease digestion, which limits their utility in many biomedical applications. Here we describe a xeno-nucleic acid system based on alpha-L-threofuranosyl nucleic acid (TNA) that is completely refractory to nuclease digestion. The use of an engineered TNA polymerase permitted the isolation of functional TNA aptamers that bind to HIV reverse transcriptase (HIV RT) with K-D 's of similar to 0.4-4.0 nM. The aptamers were identified using a display strategy that provides a powerful genotype-phenotype linkage. The TNA aptamers remain active in the presence of nuclease and exhibit markedly higher thermal stability than monoclonal antibodies. The combined properties of biological stability, high binding affinity, and thermal stability make TNA aptamers a powerful system for the development of diagnostic and therapeutic agents.