A recently developed in vitro o procedure for preparing artificial granules of microbial polyesters has been applied to a blend of poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxyoctanoate) (PHO). Density gradient analysis showed that these granules formed a single, uniform population; n.m.r. showed that the polymers retained their amorphous elastomeric state; and thermal analysis indicated that the polymers formed separate phases within these granules. These results contrast strongly with the observation that a recombinant bacterial strain simultaneously expressing PHB and PHO stores the two polymers in separate cellular granules. The implications of the in vitro findings for granule formation in vivo are discussed.