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2-Tridecanone and 2-undecanone are both found naturally in the trichomes of wild tomato plants and are important in plant resistance to herbivory. 2-Undecanone is the U.S. Environmental Protection Agency (EPA)-registered active ingredient in the commercially available arthropod repellent, BioUD(®) . The goal of this study was to examine the tick repellency of 2-tridecanone. Two-choice bioassays were conducted using 8% 2-tridecanone vs. the repellent carrier (absolute ethanol) and compared with two-choice studies using 8% 2-undecanone vs. absolute ethanol. Unfed, host-seeking adult (mixed sex) Amblyomma americanum (L.) (Acari: Ixodidae) and Dermacentor variabilis Say (Acari: Ixodidae) were used to evaluate repellency and time to repellent failure at room temperature. The present study shows in filter paper assays (0.63 mg test compound/cm(2) ) that 2-tridecanone was 87% repellent to A. americanum at 12 h after application, but had no statistically significant repellency at 15 h and 24 h, and was 72% repellent to D. variabilis at 15 h, but had no statistically significant repellency at 24 h. By contrast, 2-undecanone was 74% and 75% repellent to A. americanum and D. variabilis, respectively, at 2 h after application, but no statistically significant repellency was noted at 2.5 h and 3 h. In two-choice assays on cheesecloth, 2-tridecanone at 0.25 mg/cm(2) was 85% repellent to A. americanum 6 h after application, demonstrating its potential use as an arthropod repellent that can be used on clothing without the need for formulation. No statistically significant repellency was found at 9 h or 12 h. The potential use of 2-tridecanone as a tick repellent is discussed.