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The aim of this study was to investigate changes in the ribonucleic acid (RNA)/deoxyribonucleic acid (DNA) ratio and the fatty acid composition of cultured Octopus vulgaris (50^750 g) in relation to recent (last 30 days) speci¢c growth rate (SGR).Wild animals (80^500 g), collected in the ¢eld throughout the year (Aegean Sea, Mediterranean), were also examined for the aforementioned biochemical parameters. Octopuses were reared in a closed seawater system at three di¡erent temperatures (15, 20 and 25 1C). The octopuses were fed on squid (Loligo vulgaris). The cultured animals showed a high n-3 (33^52.9%) and n-6 (3.3^13.7%) polyunsaturated fatty acid content, but with a high variation for 22:6n-3 [docosahexaenoic acid (DHA)], 20:5n-3 [eicosapentaenoic acid (EPA)] and 20:4n-6 [arachidonic acid (AA)]. The proportion of these fatty acids (% total fatty acids) and the RNA/DNA ratio were linearly (Po0.0001) related to SGR. Speci¢cally, RNA/DNA (0.5^1.9) and AA (2.7^10.7%) increased, while EPA (10.4^19.7%) and DHA (20.8^31.9%) decreased, with increasing SGR (0.4^1.7% day À1 ). The highest levels of SGR, RNA/DNA and AA were detected in small (50^150 g) octopuses reared at 20 and 25 1C and in large (500^750 g) animals reared at 15 1C. Similar RNA/DNA levels and fatty acid percentages were found in wild octopuses. It is concluded that RNA/DNA, DHA, EPA and AA could be used as biochemical indices for predicting the growth rate of O. vulgaris.