We have examined solutions of a polystyrene-polybutadiene pentablock copolymer in 1,4-dioxane, a slightly selective solvent for polystyrene and a theta solvent for polybutadiene, with static light scattering (SLS), dynamic light scattering (DLS), and small-angle neutron scattering (SANS). The SANS data have been analyzed with the Percus-Yevick model to represent the scattering from interacting cores, approximated as hard spheres, and with a Lorentzian function to represent the scattering from unassociated and associated polymer chains. The SANS data at 25 degreesC clearly reveal interacting domains, approximately 6 nm in radius, formed by the association of the insoluble polybutadiene block in the 20% sample. The 4% sample does not show such domains, whereas the 7% sample represents an intermediate situation, with both unassociated polymer and associated polymer. At higher temperatures, the domains dissolve. The DLS data for samples with concentrations of 2-22% show two diffusive modes: a fast mode corresponding to the cooperative dynamics of concentration fluctuations and a slow mode corresponding to the diffusion of clusters. The large length-scale heterogeneities, indicated by the strong angular dependence of SLS, implies that the small microdomains of about 10-15 polybutadiene blocks are bridged by the polystyrene chains, forming large aggregates with randomly distributed crosslinks on length scales much larger than the domain size.