Triblock copolymers polystyrene-polyisoprene-polystyrene were studied in dilute solution in a selective solvent, i.e., a nonsolvent for polystyrene and a good solvent for polyisoprene. At a concentration equal to 1.6 x 10(-3) g/cm3 triblock aggregates appear. Elastic and quasi-elastic light scattering experiments, as well as intrinsic viscosity measurements, performed on these solutions show a loose structure rather than a micellar structure for the aggregates. These experimental results are also discussed from a theoretical point of view.