During July and September 1993, the longitudinal distributions of fish communities in five reaches of the River Thames were obtained by mobile acoustic surveys at night, using a BioSonics dual-beam echosounder (420 kHz) with a transducer beaming horizontally across the river. Although acoustic ranges were small (10-20 m), the total sampled volume was large. Continuous records of absolute fish densities in the water column were obtained by echo integration at 20 m or 40 m intervals along the river. The striking feature of the acoustic data is the evidence they offer for different scales of patchiness in continuous longitudinal distributions of fish targets. The scale of patches varied in density and size. Some obvious larger patches could be associated with river features (sewage outfalls) or particular events (mass emergence of insects at dawn). For other larger patches, there were no obvious causes but they were persistent in replicated runs during one night or occupied the same locality in both months. Patchiness at smaller scales was also detectable all along the river course between the larger patches. (C) 1996 International Council for the Exploration of the Sea