The entire RNA 3 (2214 nucleotides) of a chrysanthemum isolate of tomato aspermy virus (C-TAV) has been cloned and its sequence determined. C-TAV possesses two open reading frames which encode a 3a protein (277 amino acids) and a coat protein (229 amino acids). Computer-assisted comparisons were made between C-TAV RNA 3 and its predicted protein sequences and those of two other tripartite RNA viruses, cucumber mosaic virus (CMV) and brome mosaic virus (BMV). Results from this study suggest that a close evolutionary relationship exists between C-TAV, Q-CMV and BMV. Divergence of nucleotide and amino acid sequences between these viruses is not reflected at the level of the predicted secondary structure of the encoded proteins, where conservation is strong.