One-year-old rope-grown blue mussels (Mytilus edulis L.) were held in experimental lantern nets and cross-transplanted between two sea lochs (Loch Etive and Loch Kishorn) on the west coast of Scotland. Growth, mortality and shell morphology were monitored in the native and transplanted stocks for twelve months. There were significant differences (p<0.05) in growth rates between the lochs. Both native and transplanted mussels performed better (shell, live and wet meat weights) in Loch Etive than in Loch Kishorn. There were no significant effects of stock or stock*site interaction on growth parameters (p>0.05). Mortality rates were 7.3% in native and 17.1% in transplanted mussels in Loch Etive and 10.8% in native and 16.5% in transplanted mussels in Loch Kishorn. There was a significant stock effect on mortality (p<0.001), but not site effect (p>0.05). There were significant morphological differences (shell length, height and width ratios) between the Loch Etive and Loch Kishorn populations. Loch Etive mussels had higher height:length (p<0.001), width:length and width:height ratios (p<0.05) than Loch Kishorn mussels after the one year reciprocal transfer. The differences in growth rate were related to environmental factors, while morphological differences most probably resulted from genotypic differences between the stocks.