The isothermal crystallization of the poly(ethylene oxide) block in a linear diblock copolymer of poly(methyl methacrylate)-poly(ethylene oxide) with a poly(ethylene oxide) weight fraction of 0.76, has been evaluated using optical microscopy and differential scanning calorimetry. The copolymer was quenched from the melt to a range of crystallization temperatures between 289K and 316K and the crystallization monitored by observation of the increase in radius of spherulites (microscopy) or the enthalpy of fusion (calorimetry) as a function of time. Comparison experiments were also made on physical blends of the two homopolymers where the weight fraction of polyethylene oxide ranged from similar to 0.6 to 0.9. The block copolymer has an observed melting point which is 2-3K lower and the spherulite growth rate was reduced compared with the equivalent blend. The growth rates calculated from optical microscopy have been subjected to crystallization regime analysis. All three regimes are observable in the block copolymer for the supercooling conditions used here, only regimes I and II are evident for the pure poly(ethylene oxide), and for the blends regime I appears to be completely suppressed. From the regime analysis a fold surface free energy in the block copolymer of 16-20 erg cm(-2) has been obtained, which is much less than that obtained for the pure poly(ethylene oxide) or the blends. An explanation based on the favourable enthalpy of mixing with poly(methyl methacrylate) is suggested. Enthalpy of fusion data from isothermal crystallization studies on all polymers in the d.s.c. have been analysed using Avrami theory. The Avrami exponent was obtained together with an effective rate constant of crystallization. The exponent suggests that crystallization takes place via homogeneous nucleation with a spherical growth morphology, growth being controlled by the rate of attachment of molecules to the interface. By comparison of the Avrami exponent with values obtained for blends differing only in the molecular weight, the influence of melt viscosity on growth control is evident.