The spherulitic structure, growth rates and melting behaviour of blends of bacterially produced poly(D(-)-3-hydroxybutyrate) (PHB) and poly(L-lactide) (PLLA) were investigated using polarized light microscopy. Results indicated that low-molecular-weight PLLA (M(n) = 1759) was miscible in the melt over the whole composition range, whereas a blend of high-molecular-weight PLLA (M(n) = 159 400) with PHB showed biphasic separation, in accordance with calculations based on Flory-Huggins theory. Two types of spherulite were formed on cooling, relating to the crystallization of PHB and PLLA respectively. In some blends, spherulites of opposite type interpenetrated when the growth fronts met. It is proposed that lamellae belonging to one type of spherulite continued to grow in the interlamellar regions of the other type of spherulite.