Isothermal crystallization kinetics in the miscible blend system isotactic/atactic poly(P-hydroxybutyrate) (PHB) are examined by optical microscopy. The presence of the atactic component results in a reduction in spherulitic growth rate of the isotactic polymer. Analysis of the growth rates using the Lauritzen-Hoffman model shows that a regime II --> III transition is present for all blend compositions investigated (0-50 wt% atactic), and that this transition occurs at a constant temperature of 140 degrees C. The ratio of the nucleation constants for the two regimes, K-g(III)/K-g(II), is found to deviate from the theoretical value of 2.0 in the blend system. It is shown that a modified version of the Lauritzen-Hoffman equation incorporating entropic effects is unable to account for these deviations. The work of chain-folding, q, is calculated to be 5.1 kcal mol(-1) for isotactic PHB. The implications of using assumed values of the transport parameters in the kinetic analysis are discussed.