An obligate methanotrophic bacterium, strain MTS, was isolated from a methane-fed microaerobic denitrifying bioreactor. 16S rRNA and DNA–DNA hybridization analysis revealed that this organism was most closely related to Methylocystis parvus, a Type II methanotroph, belonging to the α-subclass of the Proteobacteria. The metabolism of the bacterium under microaerobic and anaerobic conditions was studied by 13C-NMR. 13C-labelled poly-β-hydroxybutyrate (PHB) formation occurred in cell suspensions incubated with 13C-labelled methane at low (5–10%) oxygen concentration. Under these conditions low levels of succinate, acetate and 2,3-butanediol were formed and excreted into the culture medium. Intracellular PHB degradation was observed in intact cells under anaerobic conditions in the absence of an exogenous carbon source during a long-term incubation of 90 days. Multiple 13C-labelled β-hydroxybutyrate, butyrate, acetate, acetone, isopropanol, 2,3-butanediol and succinate were identified as products in in vivo13C-NMR spectra and in the spectra of culture medium during the dynamic PHB degradation. The isolated obligate methanotroph clearly shows a fermentative metabolism of PHB under anaerobic conditions. The excreted products may serve as substrates for denitrifying bacteria.