The growth and survival rates of four size classes (10, 15, 20, and 25 mm) of bearded horse mussels (Modiolus barbatus) grown in net bags in Mersin Bay, Izmir, were assessed from May 2001 to May 2002. Temperature ranged 14-23 degrees C, average chlorophyll a was 3.34 +/- 0.35 mu g/l, average total particulate matter 12.43 +/- 0.68 mg/l, and average particulate organic carbon 210.23 +/- 18.00 mu g/l. Shell lengths increased 23.05, 19.76, 19.40, and 12.09 mm in the four classes (from small to large) and live weights increased 13.21, 10.46, 10.06, and 4.96 g, respectively. Survival rates ranged from 25% in the smallest size class to 87.5% in the 20 mm class (p < 0.05). Mussels grew significantly fastest in the smallest size class (p < 0.05).