Measurements of sound propagation were made in a shallow, sloping bottomed freshwater pond. The frequency responses of the pond had a highpass characteristic with a sharp cut-off frequency. Cut-off frequency of the response was inversely related to the depth of water at the shallowest transducer (either projector or receiver) and was the same whether propagating downslope or upslope (reciprocity). The relationship between cut-off frequency and depth was significantly different from that expected for propagation in a channel with either a rigid or pressure release (gas) bottom. The highways characteristic is due to modal propagation in the 'waveguide', and the effect of this environmental filtering is 30-60 dB between frequencies that propagate and those that do not. Thus, the physical constraints of this shallow-water environment on acoustic communication by aquatic animals are much greater than those measured in terrestrial environments. These constraints are discussed relative to selection for behavioural adaptations of acoustically signalling aquatic animals and are compared to similar adaptations found in terrestrial systems.