1.The dehydrocyclization of 2-ethyl-1,3-butadiene was investigated. It was shown that it can undergo ring-closure with formation of methylcyclopentadiene in 38% yield, calculated on the original hydrocarbon.2.Comparison of the results of the present investigation with the results that we obtained previously on the dehydrocyclization of alkadienes shows that C6hydrocarbons having a branched carbon-chain have an enhanced tendency to undergo dehydrocyclization, as compared with linear piperylene.