Intertidal mudflats are a dominant feature in many estuarine systems and may comprise a significant component of the feeding grounds available to fish. The Senegal sole, Solea senegalensis Kaup, 1858, is one of the most important flatfishes in the Tagus estuary (Portugal) and its juveniles feed in the large intertidal flats. Many aspects of the ecology and lifecycle of this species are unknown, including its behavioural adaptations to environmental variations like day–night and semi-lunar cycles. Such activity patterns may strongly influence its use of mudflat habitats. Two encircling nets were deployed on an intertidal flat, one in the lower and the other in the upper mudflat. Nets were placed during high tide and organisms collected when the ebbing tide left the flats dry. Sampling took place in June–July 2004, covering all possible combinations of the diel and semi-lunar cycles with six replicates. Monthly beam trawls were carried out to determine density and average length of the predators of S. senegalensis in the intertidal and subtidal areas, and sediment samples were also taken, to determine prey density in the intertidal and subtidal areas. Solea senegalensis captured were mostly 0-group juveniles. The density and average length of Crangon crangon, one of the main predators, was higher in the subtidal than in the intertidal. Prey density decreased from the upper intertidal to the subtidal area. The highest average density of S. senegalensis occurred during full moon at dawn/dusk. A semi-lunar activity pattern was detected. At spring tides abundance peaked at dusk/dawn, whilst at neap tides abundance peaked during the day. Predators’ densities over these periods were analysed and predator avoidance is discussed. During quarter and full-moon nights S. senegalensis extended its distribution over the lower and upper mudflat, but during the new moon colonisation was restricted to the lower mudflat. It was concluded that, while diel patterns of activity are well studied and are probably associated with feeding rhythms, the influence of the moon cycle, despite its importance, is a more complex phenomena that needs further investigation.