Dynamic and static light scattering and viscosity measurements were made for a polystyrene (PS)-block-poly(methyl methacrylate) (PMMA) of high molecular weight M-w = 1.53 x 10(6) (38.5 wt % PS) in mixed selective solvents of benzene/p-cymene at 30.0 degrees C, where benzene was good to PS and PMMA, while p-cymene was good to PS but poor/precipitant to PMMA. In a solvent of benzene/p-cymene = 40/60 (v/v), the block copolymer formed a monodispersed micelle and the micelle coexisted with the single copolymer (unimer) in the quiescent state. The micellization due to the weak segregation led the micelle to a thermodynamically equilibrium size of loose association, which is composed of three pieces of unimers. However, the micelle broke into unimers under the shear rate of 1700-2000 s(-1) in the capillary viscometer. This feature was caused by the flabby structure of the micelle characterized dynamically by the vast motions occurring inside the micelle. With strong preferential adsorption of p-cymene to the PS shell part and with a limited amount of benzene in the PMMA core part, the intramicellar motions became much more active than the case of usual flexible linear polymers in good solvents. The micelle structure was thus well represented by the soft-core/Gaussian chain shell model, not by the hard-core shell model.