Cultivation of aerobic and anaerobic octadecane-degrading microorganisms, as well as prokaryotic community analysis, in Niitsu crude oil artesian area were carried out. For cultivation of the aerobic microorganisms from Niitsu river soil, the bacteria closely related to known alkane-degrading bacteria have been isolated at high frequency, while a small number of bacteria belonging to unclassified taxonomic groups were also isolated. 16S rRNA gene sequences analysis revealed a microbial community with wide variety of prokaryotic taxa in the soil sample. Anaerobic cultivation of microorgaisms in the river sediment were carried out using CSTR fed by the synthetic wastewater with octadecane and acetic acid, resulting in the enrichment of an anaerobic octadecane-degradaing microbial community. Phylogenetic analysis of the microbial community, based on 16S rRNA gene sequences, indicated that microorganisms belonging to Methanosarcina, unclassified Bacteroidales and Paludibacter were frequently detected. PCR-RFLP analysis for alkyl succinate synthase gene (assA) suggests that several types of alkane degrading microorganisms were present in the reactor.