The complete nucleotide sequence of a severe isolate of Onion yellow dwarf virus contains 10,459 nucleotides encoding 3,403 amino acids, while that of an attenuated isolate contains 10,183 nucleotides encoding 3,311 amino acids. The sequence coding for the N-terminal 92 amino acids of HC-Pro was missing in the attenuated isolate. Interestingly, garlic plants growing in commercial gardens were co-infected with isolates of both types. This is the first report of a natural potyvirus deletion mutant lacking the N-terminal region of HC-Pro.