A unique sex attractant pheromone was isolated and identified from extracts of ovipositor tips from the female browntail moth (Lepidoptera: Lymantriidae). The pheromone compound, (Z,Z,Z,Z)-7,13,16,19-docosatetraen-1-ol isobutyrate, CH3CH2CH=CHCH2CH=CHCH2CH= CH(CH2)4CH=CH(CH2)6O2CCH(CH3)2, was identified by a combination of gas chromatography, mass spectrometry, and microreactions and was confirmed by synthesis. Traps baited with 5-50 μg of the synthetic pheromone, dispensed from rubber septa treated with an antioxidant and a UV stabilizer, gave male moth captures that were comparable to traps baited with three virgin females. Higher (250 μg) and lower (0.04-2.5 μg quantities of the synthetic pheromone on septa captured somewhat fewer males. Captures were the same for similar quantities of the natural and synthetic pheromone. Minor, inactive components in the tip extract were identified as a mixture of 7- and 8-pentacosanone.