Many isolates from different forest soil layers did not show appreciable growth on full strength of the conventional nutrient broth (NB medium) but grow on its 100-fold dilution (DNB medium). These isolates were divided into four types according to organic nutrient concentration in the growth medium from $1^{-1};to;10^{-4}$dilution of normal NB medium. Oligotrophic bacteria were type II and type IV which grew in $10^{-4}$ dilution of NB (1 mg C/l) medium. Sixty strains were isolated for obligate oligotrophic bacteria. Chemotaxonomic and phylogenetic characteristics of eleven isolates of acetylene-reducing (nitrogen-fixing) oligotrophic bacteria from forest soil were investigated. They showed similar characteristics: the cellular fatty acid mainly consisted of straight-chain unsaturated $C_{18:1}$ (60-84% of total fatty acids). Ubiquinone Q-10 and a high guanine plus-cytosine content(61-64 mol%) were found. Eleven isolates of nitrogen-fixing oligotrophic bacteria were found to be closely related by full 16S rDNA sequence simility and many common taxonomic traits. Analysis of full 16S rDNA sequences of eleven isolates indicated that they were more closely related to Bradyrhizobium (similarity values: 98.1-98.8%), Agromonas, Nitrobacter, and Afipia.