Rheological and dielectric behavior was examined for a 50 wt % solution of a symmetric styrene-isoprene-styrene (SIS) triblock copolymer (M = 60.2K, S content = 28%) dissolved in an I-selective solvent, n-tetradecane (C14). Dynamic mechanical tests with various strain amplitudes, gamma(0) = 0.03-0.8, were performed at various temperatures (T). At T less than or equal to 30 degrees C, the system exhibited elastic, rubberlike responses characterized with frequency (omega)-insensitive G’ being much larger than G " for small gamma(0) and slippage at the measuring assembly wall for large gamma(0). At 40 less than or equal to T/degrees C less than or equal to 80, the system showed elastic responses (similar to those at T less than or equal to 30 degrees C) for small gamma(0) and highly nonlinear, elastoplastic responses characterized with lozenge-shaped stress-strain patterns for large gamma(0) At T greater than or equal to 90 degrees C, viscous behavior was observed. These rheological changes were compared with changes in the dielectric behavior of the SIS/C14 system; the I block had dipoles parallel along the chain contour and the motion of its ends was dielectrically detected. In the rubbery regime (T less than or equal to 30 degrees C), no detectable dielectric dispersion was observed, meaning that the I block ends were anchored on rigid S domains. In the viscous regime (T greater than or equal to 90 degrees C), prominent dispersion was observed. This dispersion suggested that the I block motion was essentially free from the constraint due to the S domains and the S and I blocks were more or less homogeneously mixed with each other; In the plastic regime (40 less than or equal to T/degrees C less than or equal to 80), the system exhibited onset of dielectric dispersion (upturn of dielectric loss at low omega). This fact suggested some freedom in motion of the I block ends (in the S/I interface) at those T. In the plastic regime, the dielectric response of the SIS/C14 system remained the same at the quiescent state and under oscillatory/steady shear. A previous argument (Watanabe et al. J. Rheol. 1984, 28, 393) attributed the plasticity to strain induced motion of the I blocks pulling out the S blocks from softened S domains. However, the shear-insensitive dielectric behavior suggests that the pullout of the S blocks was not the only mechanism providing the plasticity. From this point of view, the mechanism for the plasticity of the SIS/C14 system was discussed in relation to the possible conformations of the I blocks, a bridge connecting different S domains and a loop having two ends anchored on the same domain.