NOVELTY - A feed-on-demand fed-batch method of producing a polyhydroxyalkanoate (PHA)-rich-biomass from open mixed cultures, involves: directing mixed liquor containing biomass into a fed-batch reactor including at least one biomass stimulating zone and at least one biomass maintenance zone; providing a feed containing readily biodegradable chemical oxygen demand (RBCOD) along with bio-available nitrogen (N) and phosphorus (P); adjusting the concentration of the bio-available N relative to the concentration of RBCOD to yield an adjusted feed.    USE - The feed-on-demand fed-batch method is useful for producing a polyhydroxyalkanoate (PHA)-rich-biomass from open mixed cultures (claimed).    ADVANTAGE - The method is highly economical, environment friendly and time efficient.    DETAILED DESCRIPTION - A feed-on-demand fed-batch method of producing a polyhydroxyalkanoate (PHA)-rich-biomass from open mixed cultures, involves: directing mixed liquor containing biomass into a fed-batch reactor including at least one biomass stimulating zone and at least one biomass maintenance zone; providing a feed containing readily biodegradable chemical oxygen demand (RBCOD) along with bio-available nitrogen (N) and phosphorus (P); adjusting the concentration of the bio-available N relative to the concentration of RBCOD to yield an adjusted feed with an average N to RBCOD ratio within the range of 2-15 mg-N/g-RBCOD; adjusting the concentration of the bio-available P relative to the concentration of RBCOD to yield an adjusted feed with an average P to RBCOD ratio within the range of 0.5-3 mg-P/g-RBCOD; intermittently and repeatedly-stimulating-the biomass respiration ate for at least a fraction of the biomass in the reactor by exposing the fraction of the biomass to the adjusted feed in the stimulating zone such that the biomass is stimulated on average to greater than 50% of its extant maximum respiration rate; after exposing the fraction of the biomass to the adjusted feed in the stimulating zone; transferring at least a portion of the fraction of biomass from the stimulating zone to the maintenance zone where the average RBCOD concentration is less than half the average concentration of RBCOD in the stimulating zone; and circulating the mixed liquor containing biomass back and forth between the stimulating zone and the maintenance zone such that fractions of biomass are repeatedly exposed in order to reach a high respiration rate with the adjusted feed supplied to the stimulating zone and where the biomass is otherwise maintained at a lower RBCOD concentration in the maintenance zone.