The present investigation was carried out with a view to separate bioactive constituent from Aethusa cynapium. Bioactivity guided fractionation of the anxiolytic methanol extract has led to the isolation of a novel unsaturated fatty acid. Structure of the acid characterized by UV, IR, 1H NMR, C13 NMR, MS techniques was found to be trideca-7, 9, 11-trienoic acid. Antianxiety activity was confirmed using the mCPP-induced hypolocomotion test. This new fatty acid-trideca-7,9,11-trienoic acid, isolated from A. cynapium was found to be responsible for the antianxiety activity of the plant.