Approximately one-half of the low pungency, sweet onions produced in the Vidalia onion growing area of Georgia, USA, are stored in CA (3% O2 + 5% CO2) to extend the marketing period. In some years, losses during storage caused by Botrytis allii are large. To determine if B. allii spreads in storage, sound onions, bruised onions and onions with open cuts were stored in air and CA along with onions which had been inoculated externally with B. allii. B. allii grew in the inoculated onions during storage at 1 °C in air and CA. B. allii also spread from externally infected onions to bruised onions and onions with open cuts in contact with them, but did not spread to sound onions. Pure cultures of B. allii and B. cinerea grew on potato dextrose agar (PDA) more slowly in CA than in air at 0 °C. B. allii growing on PDA did not produce spores at 0 °C, but did produce spores at 10 °C and 20 °C in air but not in CA. B. cinerea produced spores in both air and CA storage. Since spread of B. allii through the air is by spores, it is unlikely that the fungus spreads to sound onions in CA storage except by direct contact with infected onions.