We describe the performance of an inexpensive but highly sensitive light backscattering sensor (LBSS) suitable for use in deep-sea waters, where particle concentrations are typically < 0.1 mg/l. Laboratory calibrations using aluminosilicate particles and latex spheres show that the concentration-normalized backscattering of the LESS, Kbs, is greatest for particles with a diameter close to the wavelength of light emitted by the LESS (0.88 mum), declining by more than a factor of five for particles < 0.1 m or > 10 mum. Field studies indicate that in hydrothermal plumes dominated by fine-grained metal precipitates the 95% confidence interval for predicting mass concentration with an LESS ranges from +/- 0.004 mg/l for 0.03 mg/l suspensions to +/- 0.008 mg/l at 0.12 mg/l. Comparisons among a group of 19 LBSSs found between-instrument variability to reach 70%, but normalizing the raw data using a laboratory calibration procedure reduced the maximum variability to similar to 8%, Published by Elsevier Science Ltd.