With the aim of reducing costs attributed to the substrate in the production of poly(3-hydroxybutyrate-co-3-hydroxyvalerate-P(3HB-co-3HV), cultures were carried out with whey as substrate. Propionic acid, necessary for the copolymer production, was fed using two different strategies: pH regulation and pulses. The influence of these strategies on the incorporation of 3-hydroxyvalerate monomer in the copolymer using Ralstonia eutropha cultures were studied. The pulse strategy allowed a 3-hydroxyvalerate accumulation of 1.46 g in 3.93 g of polymer accumulated, corresponding to 37% of the polymer, with a yield of 0.22 mol mol(-1). During the production phase, maximum productivity was 0.17 g l(-1) h(-1). The maximum value obtained in productivity expressed in production of 3HV, was 0.014 g l(-1) h(-1). (C) 2002 Elsevier Science Ltd. All rights reserved.