5,10-Dihydrophenazine (H2Phen) was formed from phenazine (Phen) by Pseudomonas cepacia IFO 15124 in growing cultures at low oxygen tensions. Effects of culture conditions on microbial reduction of Phen with this strain were investigated. Under optimized conditions, the transformation of Phen to H2Phen by this strain gave the molar conversion yield of 30%. However, H2Phen was not detected in the culture medium when the strain was incubated with Phen with sufficient aeration.