['To investigate the effect of various single nutrient deficiencies on poly-beta-hydroxybutyrate (PHB) biosynthesis in a methane-utilizing mixed culture (dominant species Methylocystis sp. GB 25 DSM 7674).', 'Poly-beta-hydroxybutyrate accumulation experiments were performed in 7 and 70 l bioreactors and initiated by potassium, sulfur or iron deficiency. After 24 h the PHB content reached levels of 33.6%, 32.6% and 10.4% respectively. Interestingly a polymer with an ultra-high average-weight molecular weight (M(w)) of 3.1 MDa was accumulated under potassium-limited conditions. When sulfur and iron were lacking M(w) were lower by 20.6 and 41.6%. Potassium-deficiency experiments were furthermore characterized by a maximum specific PHB formation rate 0.08 g g(-1)residual biomass (R) h(-1) and a yield coefficient of 0.45 g PHB g(-1) CH(4).'] Does potassium deficiency result in accumulation of ultra-high molecular weight poly-beta-hydroxybutyrate in a methane-utilizing mixed culture?