The objective of the present paper is to describe the interrelationships that exist between the morphology of the alimentary tract, the food and feeding behaviour and the more general diurnal activity in flatfishes (Pleuronectiformes), and also to point out the places where I had to draw conclusions on behavioural aspects from circumstantial evidence instead of fact as, for instance, the feeding period and diurnal activity of psettodids and cynoglossids. In the paper I have used the classification in which flatfishes are subdivided into five families, viz., Psettodiadae, Bothidae, Pleuronectidae, Soleidae and Cynoglossidae. Weighing all available evidence on feeding and food, flatfish can be divided into three behavioural groups.

1. Fish feeders: simple intestinal loop, heavily toothed gill raker; smaller species feed also on crustaceans; day feeders, which find their prey only by sight; relatively small oifactorial lobes and large optical lobes: Psettodidae, Bothidae, Pleuronectidae of Type I.

2. Crustacean feeders : a complicated intestinal loop; less toothed gill rakers or almost lacking; they mostly feed on crustaceans, but also on molluscs and polychaetes; day feeders, which find their prey mainly by sight; olfactorial lobes medium, large optical lobes: Pleuronectidae of Type II, Cynoglossidae.

3. Polychaete-mollusc feeders: a still more complicated intestinal loop; they feed mainly on, polychaetes, but may feed also on little crustaceans. We distinguish: day feeders -with lesser toothed gill rakers than the Pleuronectidae of Type 11; they find their prey by sight, but also use olfaction; moderately developed oifactorial lobes, large optical lobes; Pleuronectidae of Type III, night feeders-gill rakers almost lacking; they find their food mainly by olfactory clues, but still possess the ability of finding their food by sight; large olfactorial lobes, small optical lobes.