This paper is the first in a series reporting on aspects of production in North Sea cod in relation to food consumption and deals with a quantitative analysis of the food of cod by means of stomach sampling. Information is provided for the southern and northern North Sea separately and although research has been concentrated in the first area the latter presents a valuable check on the more general validity of some of the findings. Weight percentages of the major food items are given in relation to narrow size classes of the cod. The gradual shifts, which are observed in the importance of different food species at increasing length, can be related to a definite size preference, which changes proportionally to the size of the cod. The total mean weight of the stomach contents is shown to increase also proportionally to the weight of the cod and no statistically significant seasonal or geographical differences in this index of feeding rate can be detected. Special attention is drawn to the amount of cannibalism and to the feeding on other commercially important species, for which also length distributions as observed in the stomachs are provided. Adult cod are shown to depend in the northern North Sea predominantly and in the southern to a large extent on the same resources as the fisheries. Some new data on digestion time as determined in aquarium experiments are given and used in addition to evidence from literature for the calculation of daily rations as a function of length.