Density-dependant growth and mortality rate of Manila clam Ruditapes philippinarum reared in net cages was investigated in Gomso Bay, Korea where unusually high mortality of clams has been reported. For the experiment, four groups of clam cages were set up with a density of 2,000 clams/m2 (group A), 1,000 clams/m2 (group B), 500 clams/m2 (group C) and 100 clams/m2 (group D). Mortality and growth of clams in each experimental cage was monitored biweekly from May 2001 to September 2001. Highest mortality in group A was observed in late August, while highest mortality of rest groups was observed in early September. In September, the cumulative mortality in group A was 99%, while it was 93.2% in group B, 91.2% in group C and 88% in group D. Shell growth rate of clams in thecages was found to be density dependent; monthly shell length increase was 0.67 mm in group A, 1.33 mm in group B, 1.63 mm in group C and 1.71 mm in group D. Our study indicated that clam growth and mortality in the Bay is density dependent and the growth and survival rate is negatively correlated with the density.