Methanotrophs have promising applications in the bioremediation of chlorinated hydrocarbons and in the production of a biopolymer, poly-beta-hydroxybutyrate (PHB). Batch bioreactor culture conditions were studied for the accumulation of PHB by methane-grown Methylosinus trichosporium OB3b, and to evaluate the effect of PHB on the bacterial capacity to degrade trichloroethylene (TCE), a common groundwater contaminant. The PHB content of the washed and lyophilized cells was measured by gas chromatography (GC), after hydrochloric acid (HCl) propanolysis. A differential GC-based assay was developed for the monomer and the polymer of beta-hydroxybutyrate utilizing 1% and 10% HCl (v/v) reaction mixtures, respectively. During bioreactor growth in a Cu-deficient modified Higgins’ medium, the cells accumulated PHB upon depletion of nitrate. A biomass yield of 3.2 g dry wt/L and a PHB accumulation of similar to 10% (w/w) were reached after 140 to 160 h, without adversely affecting the propene or TCE epoxidation specific rate given by whole cells containing soluble methane monooxygenase (sMMO). The TCE biotransformation capacity (similar to 0.25 mg TCE oxidized/mg dry cell wt) of resting cells containing similar to 10% PHB was consistently similar to 1.6-fold greater than that of cells containing only similar to 2% PHB. Higher levels (>10%) of accumulated PHB did not enhance this biotransformation capacity further. By replacing the bioreactor inlet air + CO2 mixture with pure O-2 at similar to 85 h of batch operation, a PHB accumulation of similar to 45% was achieved after 160 h, but the whole-cell sMMO activity was markedly decreased. In contrast, cells grown in a 10 mu M Cu-supplemented Higgins’ nitrate minimal salts medium (particulate MMO formation) accumulated up to 50% PHB in only 120 h, coupled with a very high biomass yield of 18 g dry cell wt/L. High PHB accumulations above similar to 20% by both the -Cu and the +Cu grown cells resulted in a decreased ratio of the electronic cell count to the absorbance at 660 nm, which is commonly used to monitor bacterial growth.