Growth, biomass and production of one year old mussels (Mytilus edulis L.) were monitored in mussel raft systems over a 15 month experimental period in Loch Etive (LE) and Loch Kishorn (LK) on the west coast of Scotland. Water temperature, salinity, seston, particulate organic matter and chlorophyll a were also determined. Salinity, amount of seston and transparency were significantly higher in LK than LE (p<0.05). Results showed that growth of mussels was relatively rapid from May to November and very slow during the rest of the year. Length increments were similar in both sites with 31.01 mm in LE and 28.75 mm in LK. Mussels reached market size (>50 mm) about two years after spat settlement. The mean specific growth rate was 5.5% in LE and 4.5% in LK. Live weight was significantly higher in LE while tissue weight was higher in LK (p<0.05) and this was attributed to heavier shell weight in LE. Tissue growth showed a clear seasonal trend, increasing during late spring and summer and decreasing in the winter. The monthly mortality was 9.1% in LE and 9.3% in LK. There were high fluctuations in biomass between sampling months, and significant differences between sites at the end of the experimental period (p<0.001). The apparent positive relationships between specific growth rate and temperature and salinity in LE, and temperature in LK, indicate that these are two main factors controlling growth when food is available in both sites.