The backward scattering coefficient bb was measured in various coastal waters with fixed-angle backscattering sensors developed by the authors. Measurements were made at four discrete wavelengths covering the spectral range 440 to 675 nm. A power law spectral dependence of bb due to scattering by particles was investigated of the form bbp((lambda) ) equals bbp ((lambda) 0) ((lambda) 0/(lambda) )(gamma , where the superscript p denotes particle scattering and (lambda) is the wavelength. The exponent (gamma) depends on the particle size distribution and composition of particles. Extensive measurements in Monterey Bay, California, showed that 0.1 <EQ (gamma) <EQ 1.1, where the lower values were in the clear, interior waters generally below 10 m. For the upper 10 m, 0.7 <EQ (gamma) <EQ 1.0, and (gamma) is congruent to 0.8. In the relatively clear Gulf of Mexico waters near Panama City, Florida, (gamma) for the upper 10 m was found to be in the range, 0.9 <EQ (gamma) <EQ 1.1, with (gamma) is congruent to 1.0. At dry Tortugas, which consists of clear coral reef water, 0.9 <EQ (gamma) <EQ 1.1, and (gamma) is congruent to 1.0. In the thick, green-brown waters of East Sound, Washington, 0 <EQ (gamma) <EQ 0.1, which thus were the most spectrally 'flat' waters measured.