Remote-sensing reflectance (Rrs ratio of the water- leaving radiance to the downwelling irradiance above the surface) with and without a vertical polarizer in front of the sensor were derived for measurements made at 90 degrees to the solar plane and in a direction 30 degrees to nadir. These measurements were carried out to see if a vertical polarizer mounted in front of a sensor would improve the Rrs results. For 28 pairs of measurements with chlorophyll- a concentrations ranging from 0.07 to 38 mg/m3, solar zenith angles from 18 degrees to 66 degrees, clear to cloudy skies, and for optically shallow and deep waters, there was no significant variation between the polarized and unpolarized Rrs values. Statistical comparisons of polarized to unpolarized results provided R2 values of 0.987, 0.987, 0.994, and 0.999 with slopes 1.007, 1.005, 0.983 and 0.998 for wavelengths at 410, 440, 550 and 630 nm, respectively. These results suggest that although the underwater light field is partially polarized, a vertical polarizer in front of a sensor will provide close results to unpolarized sensor, if the measurements were made in a direction 90 degrees to the solar plane and 30 degrees to the nadir.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.