The invention relates to an enzymatic-chemical method for obtaining polyhydroxyalkanoates (PHA), especially polyhydroxybutyrate (PHB), or the copolymers thereof, from biomass. The inventive method comprises chemically treating the biomass with a reducing agent that reduces the non-PHA cell components of the biomass. The chemical treatment is carried out before and/or after enzymatic cell disruption. The inventive method allows, unlike other cell disruption techniques, for obtaining polyhydroxyalkanoates from biomass with a relatively low PHA content (for example