The composition of the diet of Llyn Tegid trout has been determined by analysing the stomach contents, using the volume, number and occurrence methods. All three methods yield comparable results, but the volume method is the most satisfactory. The trout is an indiscriminate carnivore. The relative extent to which different animals are eaten depends mainly on their numerical representation in the fauna, though their size, habits and mobility are probably also important. In Llyn Tegid, Gammarus pulex formed 24 per cent by volume of the total food consumption. Other important animals were the caddis larvae Limnephilus lunatus and Potamophylax stellatus, the mayflies Ephemera danica (especially the sub-imago) and Leptophlebia marginata (nymph) and the isopod Asellus meridianus. Seasonal changes in the diet were considerable and appeared to be largely the result of seasonal changes in the availability of the different food organisms. From October to April, the trout fed mainly on bottom-living organisms, and from May to September on animals caught at the surface. The mean volume of food eaten increased greatly in summer to a mid-summer peak more than eight times the winter level. This was probably due to seasonal physiological changes in the trout and to a spring and summer increase in the amount of available food. A comparison of the caddis larvae eaten by the two fishes indicates that the trout is a more superficial feeder than the grayling. It is suggested that this reduces competition for food between the two species. In accordance with the findings of other workers, the trout appeared to avoid eating corixids. No important change in feeding-habits with size was found within the size-range covered by the samples (88–344 mm.), all the trout being in the insect-feeding stage.