PURPOSE: To provide a new microorganism belonging to the genus Acinetobacter having an ability to decompose linear or branched aliphatic hydrocarbons having specific carbon atoms alkylcyclohexane compounds and alkylbenzene compounds, using the compounds as only carbon sources, and useful for treating polluted seas, etc.

CONSTITUTION: A sea water sample concentrated into a concentration of 100 times by filtering sea water with a membrane filter is added to an inorganic salt medium based on natural and artificial sea waters, mixed with a linear or branched aliphatic hydrocarbon such as n-tetradecane, and subsequently subjected to an integrated culture. The obtained integrated culture solution having a n-tetradecane-decomposing ability is successively subjected to integrated cultures using a branched aliphatic hydrocarbon, pristane, a long side chain alkylcyclhoexane, and finally a long side chain alkylbenzene as only carbon sources, respectively, to provide a new microorganism belonging to the genus Acinetobactor, having an ability to decompose the ≥7C linear or branched aliphatic hydrocarbons, lkylcyclhexane compounds having ≥4C alkyl side chains, and alkylobenzene compounds having ≥8C alkyl side chains, and growing in the presence of the compounds as only carbon sources.