Greenhouse studies were conducted to determine the role ofThrips tabaciLindeman injury to onion,Allium cepaL., in the invasion of the fungal pathogenAlternaria porri(Ellis) and the subsequent development of the disease purple blotch. Individually caged onions were infested with 25 thrips per plant and inoculated 20 d later withA. porri(1,000 spores per ml of H2O). Onions were rated 6 and 7 d after inoculation. Plants infested with thrips and inoculated withA. porrideveloped more and larger lesions that coalesced as the plant matured. Older leaves were more susceptible toA. porriinfection in the absence of thrips injury. Purple blotch infection shifted to younger leaves as a result of thrips injury to onion, and more tissue was killed by thrips in the presence of purple blotch. Plant growth was negatively correlated with thrips feeding injury. Scanning electron microscopy was used to determine morphological changes in leaf tissue resulting from thrips feeding injury and the development of purple blotch. In the absence of thrips injury,A. porrienters onion leaf tissue through stomates and directly through the epidermal cell layer. When both thrips injury and purple blotch are present, leaf tissue is extremely necrotic andA. porriappears to use areas of insect damage as alternative penetration sites. Thus, the feeding wounds caused by thrips enhance entry and development of the disease by providing alternative penetration sites forA. porri.