The co-culture between Methylosinus sporium, a strictly aerobic methanotroph, and strictly anaerobic methanogens was studied in 5 L aerobic/anaerobic coupled granular sludge reactors under O-2-limited conditions. The methanogenic bacteria maintained very good metabolic activities and were able to produce sufficient methane which serviced as substrate for methanotrophic growth. Although other strictly aerobic population proliferated by two orders of magnitude after the granular sludge had been operated under O-2-limited conditions for one month, only a limited amount of the added methanotroph remained in the sludge. This result may indicate that M. sporium lacks sufficient O-2 affinity to compete with facultative bacteria for the dissolved O-2 for their growth.