Storage polymers in bacterial cells can be extracted and used as biodegradable thermoplastics. However, widespread applications have been limited by high production costs. Ln this study, activated sludge bacteria in a conventional waste water treatment system were induced, by controlling the carbon-nitrogen (C:N) ratio in the reactor liquor, to accumulate storage polymers. Specific polymer yield increased to a maximum of 0.374 g polymer/g cell when the C:N ratio was increased from from 24 to 144, whereas specific growth yield decreased with increasing C:N ratio. An optimum C:N ratio of 96 provided the highest overall polymer production yield of 0.093 g of polymer/g of carbonaceous substrate consumed, without significantly affecting the organic treatment efficiency in the waste water treatment system.