The 69 nucleotide left-terminal domain (T(L)) of the potato spindle tuber RNA viroid (PSTVd) constitutes one of its five structural elements. Due to a twofold complementary sequence repeat, two possible conformations are proposed for the T(L) secondary structure; an elongated-rod and a bifurcated form. In the present study, two T(L) mutants were designed that remove the symmetry of the sequence repeats and ensure that either the bifurcated or the elongated-rod conformation is thermodynamically favored. Imino 1H and 15N resonances were assigned for both mutants and the native T(L) domain based on 1H-1H NOESY and heteronuclear 1H-15N HSQC high-resolution NMR spectra. The NMR secondary structure analysis of all constructs establishes unambiguously the elongated-rod form as the secondary structure of the native T(L) domain. Temperature-gradient gel electrophoresis and UV melting experiments corroborate these results. A combined secondary structure and sequence analysis of T(L) domains of other Pospiviroidae family members indicates that the elongated-rod form is thermodynamically favored for the vast majority of these viroids.