The complete nucleotide (nt) sequence of the original coleus yellow viroid (CYVd) from Solenostemon scutellarioides, 'Golden Bedder', has been determined. The covalently closed single-stranded CYVd RNA molecule consists of 248 nt residues which assumes a rod-like secondary structure when folded in the model of lowest free energy. The sequence was determined by direct sequencing of RNA and from three overlapping cDNA clones. Comparison of the CYVd sequence with that of Coleus blumei viroid 1 (CbVd 1) from Germany demonstrated that they are closely related. The differences observed in the genome organization of CYVd relative to CbVd 1 were at three sites: position 25 (one U deletion), position 26 (a U was replaced by an A) and position 241 (one A insertion). The first two mutations were detected in one A-rich segment of eight nt (between positions 25 and 34). Northern blot hybridization of partially purified nucleic acids from the leaf tissue of S. scutellarioides 'Frilled Fantasy', inoculated with double-stranded cDNA, demonstrated that this fragment was infectious. These data enable CYVd to be assigned to the viroid class of plant pathogens, based on its biological properties and molecular structure. This work also gives additional support to the present classification system, in which the viroids isolated from S. scutellarioides form a distinct subgroup.