This paper reports the results from three fixed-location acoustic surveys carried out in August and September 1996 to analyse the influence of diurnal behaviour rhythms and water-level fluctuations on the migratory activities of fish in the mouth of the Regelsbrunn-arm east of Vienna, Austria. A BioSonics 105 dual-beam echosounder (420 kHz), with a circular transducer (6°/15°) beaming nearly horizontal across the arm was used, with acoustic ranges of between 20 and 30 m. All surveys were conducted during day and night. The fish density was highest during the first observation period with low water level, lowest during the second sampling period with increasing water level, and intermediate during the third period with decreasing water level. The fish density was always higher during night than during day. Diurnal behaviour rhythms influenced the fish density as well as the size-frequency distribution. Increasing or decreasing water-gauge superposed the influence of diurnal behaviour rhythms on the size-frequency distribution for immigrating fish of certain length classes. Fish within the length group 10–15 cm tended to migrate upstream during decreasing water-gauge. Fish above 400 mm total length tended to move upstream during increasing water-gauge.