The phase behavior of a poly(styrene-block-isoprene) copolymer (SI) with styrene volume fraction f = 0.18 was studied by small-angle X-ray scattering (SAXS), optical birefringence, and rheology conducted under quiescent conditions and under the influence of shear now. This sample is located at the border between ordered spheres and ordered cylinders, Near the order disorder transition, SI block copolymers with f < 0.18 form ordered spheres while those with f = 0.18 form ordered cylinders, Our sample exhibits a phase transition from ordered cylinders to disordered micelles at 70 +/-. 2 degrees C. The cylinder phase obtained by a quiescent quench is highly defective and thus cannot be detected using birefringence, However, birefringence measurements made in the presence of shear now and high-resolution SAXS measurements on both quiescent and sheared samples confirm the presence of ordered cylinder,;, The data obtained from our sample differs qualitatively from all previously published data on SI copolymers with f = 0.18. We show that the phase behavior. near the sphere-cylinder boundary, depends, on all three thermodynamic parameters: f, chi, and N and not just f and chi N, as is usually the case. This dependence on f, chi, and N is ascribed to fluctuation effects and the presence of micelles in the disordered phase, By combining the data presented here with previous results in the literature, we estimate the range of compositions over which disordered micelles are obtained in SI copolymer melts.