In Japan, a small semelid bivalve Theora f ragilis is common in the benthic communities in muddy or muddy sand bottoms of eutrophic semi-enclosed bays. The field populations of this bivalve are often exposed to the water of low dissolved oxygen with hydrogen sulfide during the summer. In this study, the tolerance of T. fragilis to sulfide (H₂S, HS^- and S^2-) was examined in laboratory experiments. Survival time of the bivalve ex-posed to anoxia even with low levels of hydrogen sulfide (mean conc.: 12-15μM; maximum conc.: 40-43μM) tended to become shorter than when exposed to anoxia alone. The lethal time (LT₅₀) was 23 hours in 12μM H₂S at 24.2°C. Survival time decreased further when exposed to moderate or high levels of hydrogen sulfide (mean conc.: 46-778μM; maximum conc.: 84-876μM). LT₅₀ values in these cases were 15.5-40.5 hours at 15.1°C and 15-21 hours at 24.2°C. The results of this study indicate that T. fragilis has a low tolerance to hydrogen sulfide, and suggest that field populations of this bivalve can barely survive under anoxic condition with hydrogen sulfide.