Poly-3-hydroxybutyrate (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) was produced using a coculture of activated sludge. When butyric acid was used as sole carbon source, PHB was produced. When valeric acid was added to the medium, PHBV was produced. The 3-hydroxyvalerate (3HV) mole fraction in the PHBV reached a maximum of 54% when valeric acid was used as sole carbon source. When the 3HV units in the co-polymer increased from 0.0 to 54.0 mol%, the melting temperature (T-m) decreased from 178 to 99 degrees C. The composition, and hence the mechanical properties, of the co-polymer produced by activated sludge can be controlled by adjusting the medium composition.