Colletotrichum acutatum J. H. Simmonds infects citrus flower petals, causing brownish lesions, young fruit drop, production of persistent calyces, and leaf distortion. This suggests that hormones may be involved in symptom development. To identify the types of hormones, cDNA clones encoding proteins related to ethylene and jasmonate (JA) biosynthesis, indole-3-acetic acid (IAA) regulation, cell-wall modification, signal transduction, or fruit ripening were used to examine differential gene expressions in calamondin (Citrus madurensis Lour) and/or `Valencia' sweet orange (Citrus sinensis Osbeck) after C. acutatum infection. Northern-blot analyses revealed that the genes encoding 1-aminocyclopropane-1-carboxylate (ACC) oxidase and 12-oxophytodienoate required for ethylene and JA biosynthesis, respectively, were highly up-regulated in both citrus species. Both gene transcripts increased markedly in petals, young fruit and stigmas, but not in calyces. The transcripts of the genes encoding IAA glucose transferase and auxin-responsive GH3-like protein, but not IAA amino acid hydrolyase, also markedly increased in both species 5 days after inoculation. The expansin and chitinase genes were slightly up-regulated, whereas the senescence-induced nuclease and ß-galactosidase genes were down-regulated in calamondin. No differential expression of transcripts was detected for the genes encoding expansin, polygalacturonase, and serine-threonine kinase in sweet orange. As compared to the water controls, infection of C. acutatum increased ethylene and IAA levels by 3- and 140-fold. In contrast, abscisic acid (ABA) levels were not significantly changed. Collectively, the results indicate that infection by C. acutatum of citrus flowers triggered differential gene expressions, mainly associated with IAA, ethylene, and JA production and regulation, and increased hormone concentrations, consistent with the hypothesis of the involvement of phytohormones in postbloom fruit drop.