Manila clam, Ruditapes philippinarum (SL; 32.74 ± 2.18 mm, TW; 8.29 ± 1.41 g, N; 450) were exposed to various concentrations of naphthalene for 20 weeks. Exposure concentrations of naphthalene established control, solvent control (ethanol), 30, 60, 90 and 120 μg Nap L-1 following results of 96h acute exposure. After exposure during 20 weeks, survival rate of the manila clam was lowed in 60, 90 and 120 μg Nap L-1 exposure group compared control group. Also, sex ratio of male was higher in 60 μg Nap L-1 exposure group (χ2 = 5.492, P < 0.05) but lowed 90 μg Nap L-1 exposure group (χ2 = 4.214, P < 0.05) and 120 μg Nap L-1 exposure group (χ2 = 30.118, P < 0.05). Gonad development was delayed in female (> 60 μg Nap L-1) and male (> 30 μg Nap L-1). Intersex was 16.28% in female, 1.68% in male. In this result, naphthalene caused survival decrease, imbalance of sex ratio, delay of gonad development of the manila clam. Also, chronic exposure to naphthalene suggested abnormal effects in reproduction of the manila clam.