Aims: To characterize a methane-utilizing poly-beta-hydroxybutyrate (PHB)-producing microbial community. Methods and Results: Three different approaches based on microbiology, analytical chemistry and molecular biology were used to determine the composition of the mixed culture. The dominant species, Methylocystis sp. GB25, represents more than 86% of the total biomass. Seven accompanying bacterial species are present in the mixed culture of which two are methylotrophic bacteria and five are utilizers of complex carbon sources. Both these groups were found to be present at the same ratio with respect to each other. Results of fatty acid analysis and PCR-DGGE fingerprints reflect the stability of the mixed-culture composition in the open system during multiple continuous growth and polymer formation processes throughout a period of 29 months. The consistently high quality of the accumulated polymer further corroborates this finding. Conclusion: The methane-utilizing mixed culture has the potential of self-regulation resulting in a stable composition even under non-aseptic conditions. Significance and Impact of the Study: Avoiding the necessity of sterile conditions, as demonstrated in this paper, is an important step towards the development of a viable large-scale process for the production of PHB using cheap substrates like methane from natural or renewable sources. This is the first report characterizing a bacterial mixed culture being used for the biotechnological production of a high-value product in an open system.