Blends of poly(D(-)3-hydroxybutyrate) (PHB) and atactic poly(methyl methacrylate) (aPMMA) were isothermally cold-crystallized from homogeneous amorphous samples obtained by quenching. X-ray techniques were employed to investigate the phase structure of the cold-crystallized blends. The long period distance and the amorphous interlamellar distance increased with increasing aPMMA content in the blends, while the thickness of the crystalline lamellae was quite constant for the blends, and higher than for pure PHB. These results indicate that during isothermal cold-crystallization the aPMMA component is segregated in the amorphous PHB interlamellar regions.