NOVELTY - Producing polyhydroyxalkanoates (PHAs) in biomass comprises feeding an organic carbon-containing substrate to the biomass by intermittently supplying the substrate to the biomass over a period of time; and controlling the frequency of the intermittent supply and amount of the intermittent supply of the substrate to the biomass such that the average molecular weight of the PHAs produced is at least 400000 g/mole.    USE - The methods are useful for producing PHAs in biomass; promoting accumulation of PHAs in biomass; producing PHA in biomass utilizing waste activated sludge and influent wastewater including RBCOD; and for producing high molecular weight PHAs in biomass (all claimed).    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are:    (1) a method of promoting accumulation of PHAs in biomass comprising feeding an organic carbon-containing substrate to the biomass by mixing substrate containing readily biodegradable chemical oxygen demand (RBCOD) with the biomass to form a biomass-substrate mixture; controlling the concentration of the RBCOD in the biomass-substrate mixture such that during PHA accumulation the concentration of RBCOD in the biomass-substrate mixture is generally maintained between 1000 mg-chemical oxygen demand (COD)/L and 10 mg-COD/L; and where the method of feeding RBCOD to the biomass and controlling the concentration of RBCOD in the biomass-substrate mixture produces PHAs having an average molecular weight above 400000 g/mole;    (2) a method of producing PHA in biomass utilizing waste activated sludge and influent wastewater including RBCOD, comprising directing waste activated sludge into a zone where the waste activated sludge includes the biomass; aerating the biomass; feeding the wastewater influent or an alternate wastewater influent to the biomass and mixing the wastewater influent with the biomass, and interrupting the feed of the wastewater influent to the biomass for some times; and feeding sufficient wastewater influent to the biomass such that the average molecular weight of the PHAs in the biomass is at least 400000 g/mole; and    (3) a method of producing high molecular weight PHAs in biomass comprising feeding an organic carbon-containing substrate to a PHA-accumulating biomass by intermittently supplying the substrate to the biomass over a period of time; and controlling the frequency and amount of the supply of substrate such that the respiration rate of the biomass does not drop more than 70% relative to the maximum respiration rate achieved by the biomass in response to the most recent supply of substrate.