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We undertook a systematic evaluation of the lipid, protein, calorie, and fatty acid composition in 35 species of fish, shellfish and molluscs commonly consumed throughout the four seasons of the year in Andalusia, Spain. Using a portion of muscle tissue the following were measured in each study unit: total lipids (extraction using Folch's method and gravimetry), protein concentration (Kjehldal's method), total calories (direct calorimetry), and composition of fatty acids (gas chromatography). The lipid, protein, and different fatty acid concentrations found are presented in table form. There was a high degree of inter-species variability in the concentration of lipids and the various fatty acids. There was also a high degree of intra-species seasonal variability in some cases. The relative proportion of fatty acids was not independent of the total concentration of lipids, independently of the season studied. This systematic study of a large group of species shows that the cataloguing of fish as 'white' or 'blue' depends especially on the time of year they are captured. For example, in spring the mackerel (Scomber scombrus), a fish considered traditionally to be 'blue' (fatty), has the same lipid concentration as the dover sole (Solea vulgaris), commonly considered to be 'white' (little fat), and the sea pike (Merluccius merluccius) has a higher lipid concentration in autumn than the jack mackerel (Trachurus trachurus). Even greater differences existed when the fish were classified according to their richness in n-3 fatty acids. These differences in the total lipid concentration and the composition of fatty acids, as well as the inter-relations between them, may, under certain circumstances, be important for the calculation of dietary calories and nutritional values, and may explain the differences found between the various tables of food composition, as well as the divergent results in epidemiological studies on the association between fish in diets and various diseases, such as diabetes or ischaemic cardiopathy.