Polarized small-angle neutron scattering studies of single-crystalline multiferroic BiFeO(3) reveal a long-wavelength spin density wave generated by ∼1° spin canting of the spins out of the rotation plane of the antiferromagnetic cycloidal order. This signifies weak ferromagnetism within mesoscopic regions of dimension 0.03 microns along [110], to several microns along [111], confirming a long-standing theoretical prediction. The average local magnetization is 0.06 μ(B)/Fe. Our results provide an indication of the intrinsic macroscopic magnetization to be expected in ferroelectric BiFeO(3) thin films under strain, where the magnetic cycloid is suppressed.