Seasonal variations in meat weights, condition indices and biochemical components of suspended cultured mussels (Mytilus edulis L.) cross-transplanted between two sea lochs (Loch Etive and Loch Leven) on the west coast of Scotland were followed during the period of their second and third growth years. All components studied showed a clear seasonal cycle with very similar patterns. There were significant differences between lochs and sometimes between stocks. Mean meat weights of mussels grown in Loch Etive was considerably higher than those in Loch Leven and when the mussels from same origin compared, condition indices of mussels in the former loch were better than those in the latter. In general, a rapid tissue restoration occurred in spring and mussels remained in good condition during summer and autumn, when protein and carbohydrate were maximal, and decreased through winter to a post-spawning minimum in spring (April) before showing rapid increases in May again. Weight losses during winter has been attributed to utilisation of carbohydrates (i.e., glycogen) reserves and depletion of both protein and lipid contents as a result of food shortage and gametogenesis. Main biochemical components were slightly higher in mussels grown in Loch Etive. Overall percentage values of the main components of ash-free dry meat varied between; protein 51.8-82.4%, carbohydrates 8.6-35.8%, lipids 2.6-12.7, while ash content of dry meat weight were 4.2-14.0%. The mean caloric value of ash-free dry meat was 5.42 kcal/g. The results indicate that cultivated mussels were suitable for marketing from May to December, particularly in Loch Etive. (C) 1998 Elsevier Science B.V.