The spherulitic and lamellar morphology of binary blends of microbial isotactic poly[(R)-3-hydroxybutyrate] (P[(R)-3HB]) (($) over bar M(n), = 650 000) and synthetic atactic poly[(R,S)-3-hydroxybutyrate] (P[(R,S)-3HB]) (($) over bar M(n) = 56 000 and 3400) were investigated by means of optical microscopy and small-angle X-ray scattering (SAXS). The spherulites of isotactic P[(R)-3HB] were volume-filled, indicating the inclusion of P[(R,S)-3HB] within the spherulites. The radial growth rate of P[(R)-3HB] spherulites decreased with increased content of P[(R,S)-3HB]. The SAXS data show that the lamellar periodicity of P[(R)-3HB] increased with the content of P[(R,S)-3HB]. The decrease in overall crystallinity of the blend was fully accounted for by addition of the amorphous P[(R,S)-3HB] component; i.e., the degree of crystallinity of the crystallizable component did not appear to change much by the presence of P[(R,S)-3HB]. These results imply that atactic P[R,S)-3HB] material is included in the amorphous regions between the lamellae under certain crystallization conditions.