Solutions of polystyrene-block-poly(butyl methacrylate) [P(S-b-BMA]-a narrowly distributed linear diblock copolymer consisting of 1200 monomeric units of styrene and 1200 units of butyl methacrylate-in 2-propanol (a nonsolvent for the S blocks and a THETA solvent (THETA = 23-degrees-C) for the BMA blocks) were studied by carring out the following measurements : dynamic (DLS) and static light scattering (SLS), transmission elctron microscopy (TEM), small-angle X-ray scattering (SAXS), and surface tension. The results demonstrate that the polymer forms micelles as soon as a very low critical micelle concentration (<4 X 10(-3) wt %) is exceeded. According to SLS these entities consist of approximately 75 unimers and exhibit an apparent radius of gyration of ca. 37 nm; this value is (in agreement with theoretical considerations) somewhat less than the sum of the radii of the S cores and of the BMA coronas, which are according to TEM 25 nm each. The hydrodynamic radius of the micelles observed in the DLS is 56 nm. It is concluded from SAXS (up to 15 wt %) of P(S-b-BMA) and from TEM that the core of the micelles does not change in size or shape; this finding is at variance with an increase of the aggregation number upon dilution concluded from SLS. The presence of micelles lowers the surface tension of the solvent measurably; the maximum reduction of ca. 1.5 mN/m is reached at a polymer concentration of 0.01 wt %, when the aggregates cover the entire surface.