Micelles of a poly(ethylene-co-propylene)-block-Poly(ethylene oxide) (PEP-PEO) copolymer in water were investigated by small angle neutron scattering (SANS), dynamic light scattering (DLS) and viscometry. The block copolymer was built of a partially deuterated PEP block and a hydrogeneous PEO block with an overall molecular weight of 11.1 x 10(3). The SANS measurements were performed at four contrasts in order to get detailed structural information about the micellar aggregates. The different scattering curves could be described very well by a spherical core-shell model with constant density in both parts. The sharp edges of the polymer density distribution were smeared by multiplication with a Gaussian in Q-space. The model yielded a core radius of R-C = 176 Angstrom and an overall micellar radius of R-M = 294 Angstrom. The smearing ranges were 11 and 32 Angstrom, respectively. The fit further yielded an aggregation number of P = 2430. DLS measurements reveal a narrow monomodal distribution of relaxation frequencies, indicating the existence of only one aggregated species. The obtained hydrodynamic radius, R-H = 339 Angstrom, as well as the radius determined by viscosity measurements, R-V = 324 Angstrom, are consistent with the result of the model fitting. The unusually large aggregation number could be explained by the large interfacial tension between water and PEP in terms of simple thermodynamic models.