Measurements of the ocean wave-radar modulation transfer function M at 4.3 GHz (C band) carried out in the North Sea are presented here. It is found that the values for M for C band lie within the same range as for X, L, and Ka bands. The measurements were made at incidence angles of 54¿ (HH and VV polarization) and 40¿ (VV polarization only). It was found that M decreases with increasing wind speed U and ocean wave frequency f. The phase of M was such that maximum backscatter occurs at the forward (leeward) face of the long ocean waves. For small dimensionless frequencies, f*=( fU)/g, the modulus of M reaches values of up to 30. This indicates that there is a source of strong hydrodynamic modulation, most probably wind induced that is as yet unexplained by existing theories.