The chloroformie extract of marine mollusc Purpura pansa was examined and the 2,3,4-trimethyl-5-(4-methyl-cyclohyxyl)-2,4-pentadiene was isolated. The structure of this compound was determined on the basis of the spectroscopic data (UV, IR, NMR, MS). The compound showed the antimicrobial activity on Staphylococcus aureus, Escherichia coli and Candida albicans.