A methane-utilizing mixed culture isolated from activated sludge by selective enrichment was found to consist of more than three interacting species:methane utilizing bacterium, methanol utilizing bacterium, and unknown bacteria which might cause denitrification. These species grew well at different hydrogen ion concentration at 45$^\circ$C. Small addition of methanol to the methane-utilizing mixed culture was shown to inhibit the methane utilizing moiety in the culture and it was shown that methanol utilizing bacterium in the mixed culture removed any inhibitory methanol. It was determined that oxygen became limiting, if a ratio of oxygen to methane in the gas phase was adopted smaller than 1.24 and vice versa. Maintenance coefficient of nitrate at methane limiting condition was determined as 0.001 (g $NO_3$/gcell.h) and at oxygen limiting condition it was determined as 0.02 (g $NO_3$/gcell.h). The difference in maintenance coefficient of nitrate clearly explained dissimilatory denitrification under oxygen limiting condition. The denitrification rate mainly depended on dissolved oxygen concentration. The maximum denitrification rate obtained at dissolved oxygen concentration of 0.25 ppm was 84 (ppm/hr).