Variation in beach seine catches was examined in two juvenile flatfish populations (Port Erin, Isle of Man and Porto Pirn, Azores) to determine the importance of diel cycles while controlling for tidal rhythmicity. At Port Erin sampling was undertaken at each low water on consecutive days over a 2‐week period in May/June and September 1991. There was no periodicity in the catches of plaice (Pleuronectes platessa) in May/June, but in September catches were higher at night. Other flatfish species (Limanda limanda, Pleuronectes flesus, Scophthalmus maximusandS. rhombus) sampled at Port Erin were nocturnal. At Porto Pirn, Azores, fish were sampled over 24 h each month from July 1989 to June 1990 and at each low water over a 2‐week period in July 1991. The only flatfish species present wasBothuspodas, which had a diurnal pattern in catches over a 1‐year period. Sampling at low water in July gave high catches at both midnight and m