Experimental results are presented which indicate a linear relation between the time-averaged amplitude of the envelope of the backscattered acoustic pulse at 192 kHz and the square root of suspended sediment concentration in the 10 to 103 mg 1-1 range. Particle sizes ranged from 2 to 140 μm. The measurements were made in a negatively buoyant, mine-tailing discharge plume in a submarine channel at depths of 60 to 90 m in Rupert Inlet, British Columbia. From the theory of acoustic backscatter from a solid elastic sphere in the Rayleigh region it is shown that if the pressure amplitude of the backscattered wave is Rayleigh distributed, then such a linear relation is to be expected. Expressions for the optimum acoustic frequency for the detection of dilute suspensions at a given range and for the minimum detectable concentration are obtained assuming a thermal noise background. The possibility that bubbles contribute to the backscatter is considered and found to be unlikely on the basis of probable bubble lifetimes.